WO2019158694A1 - A nanoporous graphene structure and method for preparation thereof - Google Patents

Abstract

The present invention provides a novel nanoporous graphene structure as well as a method for preparing thereof. In particular, the present invention relates to a novel nanoporous graphene mesh exhibiting both semiconducting and nanosieving features. The invention is also directed to the use of the nanoporous graphene structure for electronic transport and for sensing and sieving species.

Description

A NANOPOROUS GRAPHENE STRUCTURE AND METHOD

FOR PREPARATION THEREOF

Field of the invention The present invention relates to a novel nanoporous graphene structure as well as to a method for preparing thereof. In particular, the present invention relates to a nanoporous graphene structure exhibiting simultaneous semiconducting and nanosieving functionalities and novel electronic properties.

Background of the invention Graphene can be defined as a two-dimensional crystalline form of carbon, made of one-atom- thick layer of a sp² hybridized carbon atoms with a honeycomb structure, where benzene rings are fused in two dimensions giving rise to fully extended aromaticity.

The high carrier mobility of graphene, measured above 50.000 cm²/V-s at room temperature, makes it very attractive for applications in electronic devices such as field-effect transistors (FET). However, pristine two-dimensional graphene is a semi-metal and, as such, it does not possess the electronic band gap that is required to switch the current off, and elementary requirement in FETs. Therefore, the challenge of making it semiconducting, preferably with a band gap similar to the ~1 eV of Si for a reliable switching at room temperature, is crucial for applications in electronic devices. An efficient approach to induce a gap in graphene is to reduce its physical size down to the nanometer scale. At this scale, the quantum confinement of electrons opens a finite band gap that scales inversely with size. Particularly interesting for electronic applications is the case of graphene nanoribbons (GNRs), where the gap is induced by reducing only one of the lateral dimensions to the nanoscale, and charge currents can be transported along the ribbon. However, the current that runs through a single GNR is too low for electronic applications.

The driving current can be enhanced by using multiple semiconducting graphene nanochannels. This can be obtained for instance by creating a uniform array of pores or apertures to a graphene sheet with top-down fabrication methods, which leaves stripes or sections that are few nanometers wide in between the pores (e.g. as described in US9012882 B2).

The nanoporous graphene (NPG) defined as such have the added potential of acting as an atom-thick selective molecular sieve for sequencing, or for ion transport, or indeed as gas separation, or water purification. The widths required to induce electronic gaps in the range of 0.1 -3 eV lie within the 0.5-3 nm range [see Phys. Rev. Lett. 99, 186801 (2007), Nature. 514, 608-61 1 (2014)]. Optimally, FETs operating at room temperature require gaps in the range of 0.5- 1 .5 eV, which falls within the previous range. At this scale, variations in width of a single carbon atom, which is translated in 0.14 nm, can easily vary the gap as much as 100-300%. Therefore, keeping a homogeneous band gap along the whole ribbon requires an atomic scale precision in the nanostructuration of graphene. Moreover, the band gap and the type of electronic states that will define the band gap also depend on the crystallographic orientation along which the nanoribbons are cut. For GNRs with edges along the armchair direction of graphene (AGNR), the gap is induced by the lateral quantization of electrons. In contrast, for ribbons in the zigzag direction (ZGNR) the gap is driven by exchange (magnetic) interactions between edge-localized states [see Phys. Rev. Lett. 99, 186801 (2007), Nature. 514, 608-61 1 (2014)].

Likewise, selectivity in the sieving and sensing of ions and small molecules such as greenhouse gases, hydrogen, oxygen, water, hydrocarbons, nucleotides and aminoacids, require pore sizes of 0.5-3 nm size and sub-nm homogeneity in size and geometry or edge structure. A clear example of how critical the nanopore size can be for the selectivity of ion sieving is the recent work of A. Fang and coworkers, where they show that changes as small as 3-5% in the pore radius (< 0.02 nm in absolute values) can yield an order of magnitude increase in ion current [Nat. Mater. 18, 76 (2019)].

Standard top-down fabrication techniques for the fabrication of NPGs such as lithography, etching, block copolymer mask or using nanoparticles as templates (e.g. described in US9012882 B2 and US2014141581 ) are not suitable for preparing uniform structures with the required atomic scale size and precision. It is inherent to these methods to break the chemical bonds leaving behind rough edges with little control over the orientation. These methods severely compromises the edges of the graphene nanostructure since present fabrication tools are coarse relative to the -0.1 nm definition of a C-C bond. NPGs fabricated with these methods usually have an edge roughness of -2 nm, which makes it practically impossible to obtain uniform NPGs. Consequently, GNRs and NPGs fabricated by top-down methods generally possess uncontrolled and atomically imperfect edges or pores respectively, precluding detailed studies on finite size and edge effects, and impeding their technological implementation. Furthermore, top-down methods result in unavoidable contamination, deteriorating even more the properties of such structures.

Recently the bottom-up, on-surface synthesis of graphene nanoribbons have been reported, where the width and edge structure can be controlled with atomic precision [Nature 466, 470 (2010); WO 2013/072292; WO2013/175342 and WO2015/121785]. One of the most studied consists on the polymerization of 10,10’-dibromo-9,9’-bianthryl (DBBA), and cyclodehydrogenation of the resulting polymer. This leads to the formation of a 7 atom-wide armchair GNR (7-AGNR) in the case of the parent DBBA precursor, but the width can be extended to 13 atoms (13-AGNR) by using DBBA substitutional derivatives [ACS Nano. 7, 6123-6128 (2013)].

This method leads to the synthesis of isolated GNRs with atomic scale structural uniformity, which appear randomly distributed on the catalytic surface, with seldom accidental interconnections [see for instance the random orientation of GNRs in Figure 2 (c, e) of Nature 466, pp. 470-473 (2010)]. The lateral inter-ribbon fusion of armchair GNRs leads to wider armchair GNRs and eventually to the conventional, poreless graphene, but never to porous graphene structures. The lateral inter-ribbon fusion has only been occasionally observed for 7-AGNR [see for instance Figures 3a and 3c of Han Huang et al., Scientific Reports, 2: 983 (2012)], but similar results are be expected for any armchair GNRs, such as the 13-AGNR reported by Yen-Chia Chen et al. in “Tuning the Band Gap of Graphene Nanoribbons Synthetized from Molecular Precursors” in ACS Nano. 7, 6123-6128 (2013). Thus, the armchair GNRs or zigzag GNRs reported in the literature do not possess the specific ribbon edge structures to provide nanoporous in a graphene structure. Therefore, fusion of such graphene nanoribbons does not result in a nanoporous graphene structure.

Using different precursors, on-surface synthesis can lead to porous covalent structures that are not formed by arrays of ribbons but rather with arrays of quantum dots. Stephan Blankenburg et al., reported on the bottom-up on-surface synthesis of a graphene-like polycyclic aromatic carbon structure that they refer as porous graphene, even if it does not possess the extended aromaticity that characterizes graphene [ Small 6, 2266 (2010)]. Another similar porous structure has also been recently synthesized using a larger polycyclic aromatic hydrocarbon as building block [Synlett 24, 259 (2013)]. However, as opposed to graphene nanoribbons, the p aromaticity in these two structures is not extended, but localized within the individual building blocks (i.e. the phenyls of the honeycomb polyphenylene network in the structure of Figure 1 b herein, and hexabenzocoronene in the structure of Scheme 3 in Synlett 24, 259 (2013)). The electronic delocalization in these structures is limited by cross- conjugation at the building block interconnections, which in turns limits electronic mobility across the material.

None of the reported materials combine the extended aromaticity that characterize graphene nanoribbons with a distribution of nanopores within the structure. Therefore, there is still the need to provide fully aromatic semiconducting channels and sieving capability in a single nanoporous graphene structure that can be carved with atomic precision. In view of the state of the art, there is also the need to provide a method for preparing a nanoporous graphene structure capable of forming nanopores and nanoribbons with atomic precision in a reproducible manner, whose structure is suitable for simultaneous electronic transport and sieving applications.

In this respect, it is highly desirable to provide a method capable of preparing a nanoporous graphene structure with independent control on the neck and pore size that allows to decouple the engineering of electronic and sieving functionalities and facilitate the emergence of new properties (Fig. 2 herein includes the definition of pore and neck dimensions).

Summary of the invention Structure

The present invention was made in view of the prior art described above, and the first object is to provide a novel nanoporous graphene structure capable of simultaneous sieving and electrical sensing of molecular species as well as capable of gate controlling both the sieving and the electronic transport. To solve the problem, the present invention, in a first aspect, provides a nanoporous graphene structure having nanopores and nanoribbons which is characterized in that comprises monolithically-integrated parallel arrays of nanoribbons and atomically precise nanopores, wherein the nanoribbons are parallely interconnected with identical interconnections between two neighboring nanoribbons to form nanopores with atomic precision and periodic replication between two neighboring interconnections, and the nanoporous graphene structure has one- dimensional conducting channels.

The above nanoporous graphene structure has extended aromaticity at least along the longitudinal direction of nanoribbons.

In one embodiment, the nanoporous graphene structure has extended aromaticity along the longitudinal direction of nanoribbons and non-extended aromaticity along the transversal direction of nanoribbons.

The gap of the nanoporous material is exclusively determined by the width of the nanorribon units that forms it, which is turn defined by the structure of the NPG precursors, and is independent on the final NPG structure. The neck width of each nanoribbon can be lower than 5 nm, and preferably within the range of 0.25-3nm. The pore size of each nanopore can be lower and equal to 10 nm², and preferably lower than 10 nm². The nanoporous graphene structure has band gaps in the range of 0.1 to 3 eV. The range of 0.1-3 eV in the electronic band gap is covered by using nanoribbons with a width that ranges between 0.25 nm and 3 nm according to ACS Nano. 7, 6123-6128 (2013), Nature. 514, 608-611 (2014), which is included herein by reference.

Advantageously, the nanopores with atomic precision and periodic replication are suitable for gate-controlled sieving and sensing, and the one-dimensional semiconducting channels are suitable for gate-controlled electronics.

The nanoporous graphene (NPG) structure of the invention has a precise placement of the plurality of graphene nanoribbons (GNRs) into densely organized arrays, and thereby it is regarded as an array of identical nanoribbons that form multiple parallel transport channels.

Moreover, the NPG structure of the invention reveals three electronic bands: Longitudinal bands (L), Transversal bands (T) and Bay and Pore bands (P) which can be distinguished by energy-dependent conductance across the two perpendicular directions of the plurality of interconnected nanoribbons. Figures 5 and 6 detailed below show the different electronic bands of the NPG structure according to the present invention.

Advantageously, these distinguished features provide a novel NPG structure or mesh having new electronic properties which can be employed in new and combined electronic applications not previously reported in the literature.

In fact, the nanoporous graphene structure imprints a band gap, 1 D anisotropy and different type of localization in the electronic states that makes it suitable for electronic transport and molecular/ionic sensing.

The graphene structure of the present invention has been studied by combining density functional theory (DFT) and scanning tunneling spectroscopy (STS).

L and T bands originate from the C s and p orbitals. Protected within the backbone, L bands remain unperturbed in the nanoporous graphene structure due to the lack of dispersion in the transversal direction (along GC). Thus, L bands are similar to the conventional bands in straight armchair GNRs and disperse along the ribbon (along GZ). Consequently, the electronic band gap of the NPG structure, which is defined by the L bands, will also be very similar to that of the corresponding GNRs. On the contrary, T bands are localized within the wide periodic stripes, which are governed by the predetermined design of the graphene nanoribbon edges. The extension of T bands enables substantial inter-ribbon coupling and the formation of 1 D dispersing states with a similar mobility as the longitudinal ones. The resulting wave functions consist of non-interacting zigzag stripes that run across the graphene nanoribbons. The frontier bands close to the Fermi level are L bands, T bands lying higher in energy.

The origin of P bands is more exotic. Localized within the vacuum pocket defined by the multibay region, they are not related to atomic orbitals or their hybridization, as has been observed in other molecular pores of the prior art, but instead they originate from the free- electron like image potential states that are confined at the vacuum side along the GNR edge. In the lower dimension analogue, the straight graphene edge is "bent" into a periodic array of weakly coupled multibays that gives rise to rather flat superatom bands. In the nanoporous graphene structure, P states of adjacent multibays interact when they pair to form pores, leading to bonding and antibonding bands that can be detected in the DFT band structure.

Thus, novel nanoporous graphene structure reveals different transport regions. The transport regions are a region of true energy gap around the Fermi energy, another of pure longitudinal transport (L bands), and a mixed longitudinal and transversal transport region above this energy (L + T bands).

The exclusive contribution of 1 D transport channels along parallel graphene nanoribbon tracks makes the NPG structure suitable for multichannel FETs, sorting out the low current problem of single graphene nanoribbon field effect transistors while maintaining gap uniformity. The onset of transversal bands above a gating threshold provides an orthogonal, non- interacting 1 D channel. Altogether, the set of L and T bands brings to graphene the in-plane anisotropy that makes 2D materials such as black phosphorous appealing for FET, optical and sensing applications.

Finally, the presence of confined states within the nanopores makes them suitable for detection and electronic tracking in chemical and bio sensors and filters, since they could be shifted down to the Fermi level by the interaction with ions and molecules, making them detectable in transport measurements.

Thus, the NPG architecture of the first aspect of the present invention makes it suitable for using as a component in FET-sensor devices or gate-controlled sieve meshes, in addition to as conventional only-FET and only-sieves.

The NPG structure of this invention can be defined by the following properties:

The NPG is electronically anisotropic;

The NPG has a non-zero bandgap estimated in the range of -0.1-3 eV;

The NPG structure comprises monodisperse pore size and monodisperse nanoribbon neck width;

The NPG structure is a one-atom-thin mesh; The NPG structure is provided with nanopores periodically replicated having a precise structural and chemical composition. Such periodicity of nanopores can be the same or different along the longitudinal direction and along the transversal direction of nanoribbons.

The electronic and sieving properties of the NPG structure can be tuned independently by modifying the ribbon and pore size respectively.

Advantageously, the nanoporous graphene structure of the first aspect of the present invention is suitable for electronic transport applications as well as for molecular sensing and sieving.

In particular, the simultaneous electronic and sieving functionalities enables the realization of novel device architectures that combine gate control and chemical sensing, such as field-effect transistor (FET) sensors.

The electronic properties of the NPG structure or nanomesh of the invention are derived from the morphology of firstly formed nannoribbons, that is, the neck width and edge shape which will define the nanopores then formed between two neighbouring interconnections by the type of edges. It is a second aspect of the invention the NPG precursors, which can be used for preparing graphene nanoribbons (GNRs) as well as for preparing the NPG structure of the first aspect of the invention.

The present inventors have designed and synthesized a group of NPG precursors that, by means of the bottom-up synthetic technology, provides a nanoporous graphene structure with a precise control on the size and atomic structure of the nanopores and the necks separating them.

The NPG precursors is a substituted or unsubstituted polycyclic aromatic compound (PAC) or several PACs connected by sigma bonds, with two or more halide groups for chain polymerization, and n aryl groups for the formation of pores after inter-ribbon coupling. The n aryl groups are responsible of the inter-graphene nanoribbon connections formed in the NPG structure of the first aspect of the invention.

The substituted or unsubstituted polycyclic aromatic compound (PAC) having n aryl groups and two or more halide groups (X_a, X_t>, X_c, X_d) are represented by formula (I) and/or by formula (II): Formula I

wherein

X_a, X_b, independently, are Br, F, I or Cl;

ni and n₂ are independent integer number from 0 to 10; and n₂ indicate the number of phenyl rings attached in orto positions to the polyacene backbone, which can be functionalized by Ri,_n, R₂, R₂, R3,n or R_4,n and/or their C atoms substituted by Si,_n, S_2,n, S3_,n or S_4,n. rii and n₂ determine the length of pores; p-i, p2, P3, p_4, ps, pe, P7, ps, pg, and pio are integer numbers so that pi+p₂ < 9, p₃+p₄ < 9, p5+pe< 9, p7+ps< 9, p9+pio< 9 ; pi+p₂+1 , p3+p₄+1 , ps+pe+1 , P7+P8+1 , P9+P10+I indicate the number of fused benzene rings in each acene unit that form the polyacene and define the nanoribbon width. The defined nanoribbon width is within the range from 0.25 to 2.5 nm; ki, k₂, k₃, k₄, and k₅ are an integer numbers, so that k-i+k₂+k₃+k₄+k₅ < 5; ki+k₂+k₃+k₄+k₅ indicates the number of acenes along the polymerization direction;

R₂, R’₂, and R_3,n, are substituents independently selected from a group consisting of hydrogen; a linear or branched or cyclic Ci-C₂o alkyl which is unsubstituted or substituted by one or more OH, halogens, C₁-C₄ alkoxy, Ci-C₄ alkylthio or phenyl; a C₂- C₁₀ alkyl which is interrupted by one or more non-consecutive O; halogens; OR_a; SR_a; CN; NO2; COOR_a; OCOOR_a; OCONR_aR_b; OCOR_a; C1-C12 alkoxy; C1-C12 alkylthio; NR_aR_b; aryl or heteroaryl; wherein R_a and R_b independently of each other, are hydrogen, linear or branched or cyclic C-i-Cs alkyl, aryl or heteroaryl; and

Ri_,n and R_4,n are substituents independently selected from a group consisting of hydrogen; a linear or branched or cyclic C₁-C₂₀ alkyl which is unsubstituted or substituted by one or more OH, halogens, Ci-C₄ alkoxy, Ci-C₄ alkylthio; a C₂-C₁₀ alkyl which is interrupted by one or more non-consecutive O; halogens; OR_a; SR_a; CN; NO₂; COOR_a; OCOOR_a; OCONR_aR_b; OCOR_a; C₁-C₁₂ alkoxy; C₁-C₁₂ alkylthio; NR_aR_b; aryl or heteroaryl; wherein R_a and R_b independently of each other, are hydrogen, linear or branched or cyclic Ci-Cs alkyl, aryl or heteroaryl; and

Si_,n, S2_,n, S_3,n and S₄,_n are carbon or heteroatoms independently of each other; Heteroatoms are nitrogen, phosphor or silicon, with the proviso that nitrogen and phosphor heteroatoms do not accept any substituent R attached to it; with the proviso that if is 0, and/or n₂ is 0, then R₂ and/or R’₂, respectively, is/are aryl or heteroaryl.

In a particular preferable combination of formula (I):

P1 = P2=P5=P6=P9=P10=0;

P3=P₄=P7=P8=1

ki= k₃= k₅=0

k₂= k₄=1

ni=n2=1 -3

Rl,n⁼ R3,n⁼ R4,n⁼R2⁼R 2⁼H (fO P=1-3)

Sl _n ⁼ S2,n⁼ S3,n⁼ S4,n⁼ C (foi 11=1-3)

This leads to NPG precursors having the formula (h):

wherein

X_a and X_b independently are Br, F, I or Cl.

NPGs have been synthesized using precursors of formula (h) for n=1 (Figure 4) and n=2 (Figures 8-10).

In another particular preferable combination of formula (I):

P1 = P2=P5=P6=P9=P10=0

P3=P4=P7=P8=1

ki= k₃= k₅=0

k₂= k₄=1

ni=ri2=1

R_å ⁼R4,I⁼ H (RI,I , R_3,I = null)

S_å,i ⁼ S4,i⁼ C

Si,i⁼ S3,i⁼ N

This leads to NPG precursors with the formula (l₂):

wherein

X_a and X_b independently are Br, F, I or Cl.

In a still another particular combination of formula (I):

P1 = P2=P5=P6=P9=P10=0

P3=P4=P7=P8=1

ki= k₃= k₅=0

k₂= k₄=1

ni=ri2=2

Rl,1⁼ R3, 1⁼ RI,2⁼ R2⁼ R3,2⁼H (R4,2 ⁼ HUll)

R4,I= C5H5N (pyridine)

SI,1= Szi⁼ S3, 1⁼ S4, 1⁼ Si, 2= S_2,2=S3,2=C

S_4,2= N

This leads to NPG precursors with the formula (I3):

wherein

X_a and X_b independently are Br, F, I or Cl. Formula II

wherein

X_a, X_b, X_c, X_d, independently, are Br, F, I, Cl, or H;

ni and n₂ are independent integer number from 0 to 10; and n₂ indicate the number of phenyl rings attached in orto positions to the polyacene backbone that can be functionalized by Ri,_n, R2,n, R3, R 3, R_4,n or R_5,n and/or their C atoms substituted by Si,_n, S_2,n, S3,n, S_4,n or Ss,n. ni and n₂ determine the length of pores; In Formula II ni and n₂ can be 0 since the benzene ring of the benzoacenes can play the role of the linking group; pi, and p_2, are integer numbers so that pi+p₂< 9, pi+p₂+3 indicate the number of fused benzene rings in each acene unit that form the polyacene; k is an integer number from 0 to 3 ; k+2 indicates the number of acenes along the polymerization direction and defines the nanoribbon width. The defined nanoribbon width is within the range from 0.7 to 1.6 nm.

Ri,_n, R_2,n, R3, R3, R_4,n and R5,_n are substituents independently selected from a group consisting of hydrogen; a linear or branched or cyclic Ci-C₂o alkyl which is unsubstituted or substituted by one or more OH, halogens, Ci-C₄ alkoxy, Ci-C₄ alkylthio or phenyl; a C₂-Cio alkyl which is interrupted by one or more non-consecutive O; halogens; OR_a; SR_a; CN; N0₂; COOR_a; OCOOR_a; OCONR_aR_b; OCOR_a; Ci-Ci₂ alkoxy; Ci-Ci₂ alkylthio; NR_aR_b; aryl or heteroaryl; wherein R_a and R_b independently of each other, are hydrogen, linear or branched or cyclic C-i-Cs alkyl, aryl or heteroaryl and

Si,_n, S2,n, S3,n S₄,n and Ss,n are carbon or heteroatoms independently of each other; Heteroatoms are nitrogen, phosphor and silicon, with the proviso that nitrogen and phosphor heteroatoms do not accept any substituent R attached to it.

In a particular preferable combination of formula (II):

P1 = P2=1

k=0

ni=n₂=0

Rl,1⁼ R2,1⁼ R3⁼R4,1⁼ R5,1⁼ H

SI,1= S2,1⁼ S3,1⁼ S4,1⁼ Ss,1 ⁼ C

This leads to NPG precursors with the formula (lh)

wherein

X_a, Xb, Xc, Xd, independently, are Br, F, I, Cl, or H.

In another particular preferable combination of formula (II):

P₁ = P₂=0

k=0 Pi=h2=1

Rl,1= Rå,1⁼ R3⁼R4,1⁼ R5,1⁼ H

SI,1= S2,1⁼ S3,1⁼ S4,1⁼ Ss,1 ⁼ C This leads to NPG precursors with the formula (II2):

)

wherein

X_a, X_b, X_c, X_d, independently, are Br, F, I, Cl, or H;

The variables n, p and k can be modified within the above range values depending on the capability to deposit the monomers intact over the surface. The more massive the molecules are, the higher the temperature is required for their thermal evaporation, thus the more likely their unwanted fragmentation during the thermal evaporation. Different evaporation techniques such as spray, injection pulse or other methods of deposition can be used to ensure an adequate deposition of the NPG precursor.

In a third aspect, the present invention is also directed to one of the following nanoporous graphene structures (A-F), which have been formed from one of the NPG precursors described above of Formulas (I) and (II):

Method

In a invention is directed to a practical method capable of forming nanomeshes having nanoribbons and nanopores with a neck width and a pore size in a nanometer scale and in an atomically reproducible way.

According to the fourth aspect, the invention provides a method for preparing the nanoporous graphene structure of the first aspect, based on the on-surface bottom-up technology, which is characterized in that it comprises the following steps: a. providing at least one NPG precursor, which has a substituted or unsubstituted polycyclic aromatic compound with n aryl groups and two or more halide leaving groups represented by the formulas (l)-(ll) defined in the second aspect of the invention, on a solid substrate;

b. polymerization of the NPG precursors of step a) to form at least one polymer on the surface of the solid substrate;

c. at least partial cyclodehydrogenation of one or more polymers obtained in step b) to convert it into graphene nanoribbons;

wherein the method further comprises:

d. at least partial covalent cross coupling of one or more graphene nanoribbons formed in step c) to laterally interconnect the graphene nanoribbons thereby forming the nanoporous graphene structure; and

e. optionally, transferring the nanoporous graphene structure obtained in step d) onto other substrates

Steps a-c leads to the formation of GNRs that, for the specific NPG precursors claimed in the present invention have the particular edge structures required for the latter formation of pores. These are formed in an additional step (step d) where the lateral intercoupling of nanoribbons leads to a completely new material, the nanoporous graphene structure claimed in the attached claims.

The present invention provides an advantageous method capable of independently controlling the neck width and the pore size in the NPG structure. The neck width is given by the width of the nanoribbons that are formed in steps a) to c) above, whereas the pore size is defined by the edge topology of the resulting nanoribbons and their coupling in step d) above.

This method allows to prepare uniform NPG structure with monodisperse neck width and pore size because the edge configuration of nanoribbons and the inter-ribbon coupling can be controlled with interatomic bond dimension level accuracy of - 0.1 nm.

According to the method of the fourth aspect of the invention, the neck width of nanoribbons can be controlled, so that it is suitable for finely tuning the size of the band gap and therefore the on-off ratio of FETs that are fabricated with the corresponding NPG. Moreover, the size and periodicity of nanopores can be controlled as well, so that it is also suitable for finely tuning the total area of graphene structure which constitutes the conducting channels for electrons and, therefore, provides a way to control the driving current, transconductance and frequency response, as well as the sieving selectivity and permeability of a device containing the NPG structure of the first aspect of the present invention.

To prepare the NPG structure, different types of solid substrates can be used for depositing the precursor monomer compound and subsequent polymerization on its surface. The solid substrate on which the precursor monomer compound is deposited can have a metal surface such as for example Au, Ag, Cu, Al, W, Ni, Pt, or a Pd surfaces. The surface can be completely flat or patterned. Preferably, the solid substrate has a flat surface. Such patterned or stepped surfaces and manufacturing methods thereof are known to the skilled person. On patterned surfaces the growth of graphene nanoribbons can be directed by the surface pattern.

The solid substrate can also have a metal oxide surface such as silicon oxide, silicon oxynitride, hafnium silicate, nitrided hafnium silicates (HfSiON), zirconium silicate, hafnium (di)oxide and zirconium dioxide, or aluminum oxide, copper oxide, iron oxide, or in strontium titanate, lead titanate, barium titanate.

The surface can also be made of a semiconducting material such as silicon, germanium, gallium arsenide, silicon carbide, and molybdenum disulfide, molybdenum diselenide, molybdenum diteluride, tungsten disulfide, tungsten diselenide, and phosphorene. The surface can also be a material such as boron nitride, sodium chloride, or calcite.

The surface can be electrically conducting, semiconducting, or insulating. The surface can be non-magnetic or magnetic (ferro- or anti-ferromagnetic).

Although the scope of the present invention is not restricted to a particular solid substrate, it is preferable a solid substrate of Au (1 11 ). - Step a) -

The deposition of the NPG precursor on the surface of the solid substrate can be carried out by any process suitable for providing organic compounds on a surface. The process may e.g. be a vacuum deposition (sublimation) process, a solution based process such as spin coating, spray coating, dip coating, printing, electrospray deposition, or a laser induced desorption or transfer process. The deposition process can also be a direct surface to surface transfer.

Preferably, the deposition is carried out by a vacuum deposition process. More preferably, it is a vacuum sublimation process. The vacuum can be in the range of 1 0^"1 to 1 0^{"1 1} mbar. - Step b) -

Then, the method comprises the polymerization step, wherein the NPG precursor deposited on the surface polymerizes so as to form at least one polymer on the surface of the solid substrate. Preferably, Ullmann coupling between aryl halides of the NPG precursor is performed, whereby aryl radicals are formed and subsequently coupled by means of C-C bond formation.

Appropriate conditions for effecting polymerization of aryl halides of the NPG precursor are generally known to the skilled person.

Preferably, the polymerization step is induced by thermal activation. However, any other energy input which induces the polymerization of the NPG precursor such as electromagnetic radiation, and electric currents or fields can be used as well. The activation temperature is dependent on the selected substrate and the NPG precursor so the annealing temperature can be within the range of -100 to 500°C.

Preferably, the polymerization is carried out by annealing at a temperature T1 , wherein T1>0°C. Preferable temperature is within the range 100-300°C.

Optionally, the deposition step and the polymerization step can be repeated at least once before carrying out the next step of cyclodehydrogenation.

- Step c)-

Then, the method comprises the cyclodehydrogenation step, wherein the polyacenes (and the ni=n₂=1 phenyls in Formula I) cyclize, whereby defining a modulated neck width and bays in the edge structure of the nanoribbons and leaving available C-H bonds at their aryl edges.

In general, appropriate reaction conditions for cyclodehydrogenation are known to the skilled person.

Preferably, the cyclodehydrogenation step is induced by temperature. Preferable, the cyclodehydrogenation reaction is carried out by annealing at a temperature T2, wherein T2 > T 1. Preferably, T2 is within the range of the employed T 1 value in the previous step and up to 420°C.

- Step d) -

Then, the method comprises a covalent cross coupling step, wherein the graphene nanoribbons laterally interconnect, and thereby forming atomically identical nanopores. The inter-ribbon cross coupling preferably takes place by means of a dehydrogenative aryl-aryl coupling.

In general, appropriate reaction conditions for dehydrogenative cross coupling are known to the skilled person. Preferably, the dehydrogenative cross coupling step is induced by temperature. The dehydrogenative cross coupling step is preferably carried out by annealing at a temperature T3, wherein T3 > T2, being T2 > T1. Therefore, preferable T3 is within the range of the employed T2 value in the previous step and up to 700°C or up to a temperature at which the graphene structure is thermally degraded. Surprisingly, the selective dehydrogenative cross coupling step using monomers of formula h leads to a yield higher than 98%.

In a preferable embodiment, the method employs 10,10'-di/7a/o-2,2'-n-phenyl-9,9'- bianthracene of formula (h) as NPG precursor in step a). In this particular embodiment, the NPGs obtained with formula h and ni=n₂=1 , shows 1 D longitudinal transport within a gating window of ± 1 V, i.e. within the optimal range for room temperature FET applications.

A preferred method comprises to carry out steps b) to d) with temperature. Particularly, with electromagnetic radiation, electric currents or fields. Preferably, the steps b) to d) will be induced by thermal activation, in the range of -100°C to 700°C, wherein T1 <T2<T3 corresponds to the temperature of steps b)-d), respectively. In a preferable embodiment, in step a), the substrate’s surface is selected to induce a unidirectional orientation of the polymers.

Preferably, the polymers obtained in step b) and GNRs obtained in step c) are unidirectionally aligned by means of guiding of a patterned substrate surface.

In a particular embodiment, at least two different NPG precursors according to the second aspect of the present invention are employed in the deposition step. According to this embodiment, two or more different NPG precursors, preferably having similar reactivity, are deposited on the surface of the solid substrate, followed by inducing polymerization so as to form a co-polymer. Subsequently, the cyclodehydrogenation reaction is carried out leading to a plurality of graphene nanoribbons in parallel disposition. In this particular embodiment, the NPG structure thus obtained comprises discrete and monodisperse pore size and discrete and monodisperse neck width, the discrete distribution results from the use of more than one different precursor monomer. It is an additional object of the present invention the graphene nanoribbons obtainable by the method according to the fourth aspect of the invention wherein the method comprises steps a) to c). Applications

In a further aspect, the present invention is also directed to the use of the NPG structure according to the first and fourth aspects of the present invention for individually or simultaneous sieving and electrical sensing of molecular species that are conducted across the pores. The selective passage of ions or molecules through the NPG membrane of the first aspect of the present invention can be determined by the size and morphology of pores and the electrostatic interaction between the ions or molecules and the pores. The latter can be controlled by electrically gating the membrane. Moreover, the changes in the electric signals measured in the NPG structure as ion or molecules pass through or get anchored at the nanopores can be used to detect molecules. In a further aspect, the present invention is also directed to the use of the NPG structure of the first and fourth aspects of the present invention for gate-controlled electronic transport. In an embodiment, the NPG structure can be used as an active component in a field-effect transistor (FET) device. Then, the NPG structure of the invention is suitable for a FET-sensor device. The NPG structure of the invention is also suitable for a gate-controlled sieve mesh. Advantageously, the current limit and transconductance in a NPG is multiplied by the number of nanoribbons or channels that it contains, and the on/off ratio is improved accordingly.

The filtering and semiconducting functionalities of the new NPG structure of the invention can be combined for use in a multifunctional device where, for example, sensing is carried out by the FET actuation, and/or the filtering is controlled by electric gating the nanoporous graphene structure.

Definitions

In the present invention, the term“graphene nanoribbon” means a stripe of graphene where one dimension is significantly different than the other one, where the shortest dimension is under 10 nm width. The aromaticity along the longest direction must be extended. In the present invention, the term“periodicity of nanopores” means the distance between two equivalent positions in neighbouring nanopores in the two lateral directions. There can be two different periodicities, one along and one across the long axis of the GNR (longitudinal and transversal, respectively, see Fig. 2). Periodicity can be also formally defined by the modulus of the two vectors that define the crystallographic unit cell. Replicating the unit cell structure by using integer numbers of the unit cell vectors the periodic structure can be obtained.

In the present invention, the term“neck width” of nanoribbons means the smallest edge-to- edge distance between two neighbouring nanopores in the mesh or structure (see Fig. 2). In the present invention, the term“extended aromaticity” means that the cyclic conjugated p system is delocalized throughout the structure, and“non-extended aromaticity” means that the cyclic conjugated p system is limited either to a single or a few honeycomb units of the structure.

In the present invention, the term“monolithically-integrated parallel arrays of nanoribbons” means that the structure can be considered as an integrated circuit of coupled components that are all identical, wherein the components are the nanoribbons that are formed in parallel arrays by means of the step c of the method defined in the fourth aspect of the present invention. That is, the nanoribbons are parallel interconnected with identical interconnections between two neighboring nanoribbons to form nanopores having atomic precision and periodic replication between two neighboring interconnections.

In the present invention, the term“atomically precise” means precise control with interatomic bond dimension level accuracy (0.1 nm) over the edge roughness, size of nanopores and meshing orientation of the graphene nanoporous structure.

In the present invention, the term“monodisperse pore size” of NPG means that all pores have identical size for each NPG type within the level accuracy of 0.1 nm.

In the present invention, the term“discrete monodisperse pore size” of NPG means that the pore sizes of the NPG can only take on a certain number of monodispersed size values.

In the present invention, the term“monodisperse neck width” of graphene nanoribbons means that all graphene nanoribbons have identical width for each NPG type within the level accuracy of O.l nm.

In the present invention, the term“discrete monodisperse neck width” of graphene nanoribbons means that the graphene nanoribbons can only take on a certain number of monodispersed neck width values.

In the present invention, the term“gate control” of the transport properties of NPG means that the charge current is modulated by an electrostatic gate terminal.

In the present invention, the term“gate control” of the sieving of NPG means that the passage of atoms, molecules, or ion is modulated by an electrostatic gate terminal.

In the present invention, the term“Heteroaryl” refers to a stable 5 to 15 membered-ring constituted by carbon atoms and 1 to 5 heteroatoms selected from nitrogen, oxygen and sulphur, wherein at least one of the rings is aromatic, preferably a 4 to 8 membered-ring constituted by one or more heteroatoms, and more preferably a 5 to 6 membered-ring with one or more heretoatoms. For the purposes of this invention, heteroaryl groups can be a monocyclic, bicyclic or tricyclic systems, which can include fused rings. The heteroaryl ring can be substituted by one or more substituents selected from the group consisting of a halogen atom, an alkoxy group, an alkyl group, a thioalkoxy group, a cyano group, a nitro group or CF3. Examples of such heteroaryl include, for example, furan, pyrrole, thiophene, imidazole, oxazole, pyridine, pyrazine and pyrimidine.

In the present invention, the term“Aryl” refers to an aromatic hydrocarbon with 6 to 10 carbon atoms, such as phenyl or naphtyl, optionally substituted by one or more substituents selected from the group consisting of a halogen atom, an alkoxy group, a cyano group, a nitro group, a thioalkoxy group, an alkyl group or CF3.

In the present invention, the term "(Ci-Ci2)Alkyl" means a straight or branched hydrocarbon chain, consisting of carbon and hydrogen atoms, without unsaturations, of 1 to 12, preferably eight, more preferably one to four carbon atoms, which binds to the rest of the molecule by a single bond, which may be optionally isotopically labelled so that one or more hydrogens are substituted by deuterium (2H) or tritium (3H) and/or one or more carbons are substituted by 1 1 -carbon (1 1 C), 13-carbon (13C) or 14-carbon (14C) optionally substituted by one or more substituents selected from the group consisting of a halogen atom, a (C1-C12) alkylcarboxy group, a (C6-C10) arylcarboxy group, a (C1-C12) alkoxyl group, a cyano group, a nitro group, a (C1-C12) thioalkoxyl group, a (C1-C12) heteroalkyl group, a (C3-C15) heterocyclic group or CF3. Examples of alkyl groups include, without limitation, methyl, ethyl, n-propyl, i-propyl,n-butyl, t- butyl, n-pentyl, cyclopropyl, etc.

In the present invention, the term "(Ci-Ci2)cyclic alkyl" means a closed hydrocarbon chain consisting of carbon and hydrogen atoms, without unsaturation, of 1 to 12, preferably eight, more preferably five to eight carbon atoms, which binds to the rest of the molecule by a single bond, which may optionally be isotopically labelled so that one or more hydrogens are substituted by deuterium (2H) or tritium (3H) and/or one or more carbons are substituted by 1 1 -carbon (11 C), 13-carbon (13C) or 14-carbon (14C).

Brief Description of the Drawings

Figure 1a depicts a schematic view of the graphene nanoribbons of the prior art showing the armchair graphene nanoribbons (AGNRs) prepared from 10,10’-dibromo-9,9’-bianthryl (DBBA) via polyanthracene on Ag(1 11 ), in 7-, 14- and 21-AGNRs.

Figure 1 b depicts a schematic view of the graphene-like polycyclic aromatic carbon structure with restricted p aromacity of the prior art showing the intermolecular coupling and the honeycomb network named as porous graphene prepared from deposition and dehalogenation of (cyclohexa-m-phenylenes) CHP molecules on Ag (1 11 ) at room temperature.

Figure 2 depicts a schematic view of the nanoporous graphene structure according to an embodiment of the present invention showing different periodicities along the transversal and longitudinal direction of the structure, in which the nanopores and the nanoribbons are atomically replicated throughout the nanoporous graphene structure.

Figure 3 depicts the method steps carried out according to a preferred embodiment of the present invention [steps a)-d)].

Figure 4 depicts a Laplacian filtered topographic close-up image of the NPG structure obtained according to Example 1 , showing a regular and parallel array of nanoribbons and identical pores with very few defects.

Figure 5 depicts two graphs showing the band structure calculated by DFT for individual 7/13- AGNRs (left) and the NPG (right). Examples of longitudinal (L), transversal (T) and bay/pore (P) bands are guiding lines respectively.

Figure 6 depicts the wave functions at G for each of the L, T and P bands shown in Figure 5. Figure 7 depicts two graphs showing the dl/dV spectra acquired at the multibay edge of a 7/13-AGNR (reference A), and the dl/dV spectra acquired at the peripheral multibay and a pore region of an NPG (reference B). In both, spectra acquired on Au(111 ) is added in shaded color.

Figure 8 depicts a topographic close-up image of the NPG structure B1 obtained from using precursors h for n=2. Figure 9 depicts a topographic close-up image of the NPG structure D1 obtained from using precursors h for n=2.

Figure 10 depicts a topographic close-up image of the NPG structure C1 obtained from using precursors h for n=2.

Detailed Description of the Invention Hereinafter, the best mode for carrying out the present invention is described in detail. The best mode is described making reference to Figure 3. The nanoporous graphene structure of the present invention can be prepared following a method that relies on the hierarchical control of three thermally activated reaction steps, labelled as T1-T3, graphically represented in Figure 3, in which the nanoribbons and pores with nanometer size, atomic-scale uniformity and long-range order are formed in separate steps.

According to Figure 3, following the NPG precursor deposition (reference A), graphene nanoribbons (GNRs) are synthesized (reference B, T1 ), preferably by surface-assisted Ullmann coupling of NPG precursors into polymer chains. Then, the cyclodehydrogenative aromatization of the intermediate polymeric chains into GNRs is carried out (reference C, T2). The final step (reference D, T3) interconnects GNRs laterally in a reproducible manner via a highly selective dehydrogenative cross coupling. This step requires a careful design of the NPG precursor, which defines the edge topology of the resulting GNR that is necessary for a high yield and selectivity of the cross coupling reaction.

The presence of two equivalent C-H³ in the end of edge of the nanoribbon, labelled as A and B in reference C, can lead to either A-B or B-A bonding configurations, resulting in upwards and downwards oriented pores of identical morphology.

The preferable NPG precursor engineered by the present inventors has formula (h). The phenyl substituents at these positions are the key element for the promotion of the inter-GNR connections that then lead to the nanoporous graphene (NPG) structure shown in Figure 3 (reference D).

Examples

Hereinafter, an embodiment of the present invention is described and specifically with reference to the Figures, which however are not intended to limit the present invention.

Example 1 In this example, the selected NPG precursor was 10,10'-dibromide-2,2'-phenyl-9,9'- bianthracene (DP-DBBA) of formula (h). This example has been schematically illustrated in the synthetic hierarchical path shown in Figure 3.

DP-DBBA monomer as precursor (reference A) is deposited on a solid substrate of Au (11 1 ). At T1 =200°C, DP-DBBA is debrominated and the radical carbon atoms cross couple to form polymer chains (reference B). At T2=400°C, an intramolecular cyclodehydrogenation leads to the planar graphene nanoribbon. The cyclization of the phenyl substituent modulates the width of the GNR with pairs of 7 and 13 C atom wide sections, forming multibay regions that consist of three conjoined bay regions, and leaving three types of C-H bonds at the edge (H^1-3). Each type has two equivalent positions, as represented for H³ with A and B labels (reference C). Finally, at T3=450°C the GNRs are interconnected from the H³ bonds via dehydrogenative cross coupling, giving rise to the nanoporous graphene structure (reference D). The A-B or B-A bonding combinations give rise to identical pores with different orientations.

Structures obtained in each step of the hierarchical synthetic route of Example 1 can be resolved using scanning tunneling microscopy (STM) topographic images. After deposition with the sample kept at room temperature and annealing to T1 = 200°C, monomers undergo debromination to form the corresponding aryl radicals which are subsequently coupled by means of C-C bond formation (T1 ). The resulting polymeric chains exhibit the characteristic protrusion pairs with a periodicity of 0.84 nm and an apparent height of 0.31 nm that arise from probing the high-ends of the staggered bis-anthracene units of the monomer with STM. The chains, with dimensions up to 150 nm, predominantly align in close-packed ensembles along the zig-zag orientation of the herringbone reconstruction of the Au(1 1 1 ) surface. Both the extraordinary length of the polymeric intermediates and their parallel alignment are crucial ingredients for the high yield and long-range order observed in the final step T3.

Annealing to T2 = 400°C triggers the intramolecular cyclodehydrogenation, giving rise to the aromatization of the chain and the corresponding reduction of apparent height to h = 0.18 nm, characteristic of GNRs. The nanoribbons appear dispersed as individual chains, yet they maintain a predominant parallel alignment along the zig-zag orientation. From high-resolution image, it can be seen that the cata-fused benzene rings arise from the cyclization of the phenyl substituent bring in a periodic modulation of the width, with consecutive pairs of 7 and 13 C atoms defining multibay regions made of three conjoined bays shown in Figure 3 (reference C). This particular edge structure of the nanoribbons, labelled as 7/13-AGNR hereafter, defines both the morphology and size of the corresponding pores in the nanoporous graphene (NPG), and its electronic structure.

The aryl-aryl inter-ribbon connection is induced by further annealing to T3 = 450°C. The GNRs tend to merge together and connect laterally from each of the fused benzene rings, forming a porous graphene nanomesh. Its coincidence with the NPG structure depicted in Figure 3, see reference D, reveals that the inter-ribbon coupling occurs via a selective C-H³ bond activation. The activation of specific C-H bonds in polycyclic aromatic hydrocarbons is non-trivial due to the presence of multiple quasienergetic bonds (three in the case of the 7/13-AGNR, labelled as H^{1 3} in Figure 3). In step T3, the selectivity in the C-C bond formation between GNR is driven by the easy accessibility to the radical formed after the C-H³ bond cleavage as opposed to the steric hindrance associated with the radicals formed in after the C-H¹ or C-H² bond cleavage. Another remarkable milestone is the long-range order achieved. To date, the observation of selective intermolecular aryl-aryl coupling has been limited to small supramolecular structures. The hierarchical strategy of the present method allows to set the long-range order in step T2, where the length of prealigned GNRs represent the size limitation of the NPG. The high yield and remarkable selectivity of this coupling mechanism can be best appreciated when the surface is saturated with polymeric chains by depositing the precursor with the substrate kept at T = 200°C resulting in a coupling yield close to 100%, where every GNR is integrated in a NPG domain.

Following this method, NPG sheets as large as 70 x 50 nm², with atomically reproducible pores of 0.9 x 0.4 nm², and a characteristic defect concentration of ~2% have been taken-off. The density of nanopores can be as high as 480.000 pores/mhΊ².

The constant current STM images performed during the method when obtaining the nanoporous graphene structure reveal the internal structure in each case. The high resolution images were obtained by using a CO- functionalized tip in constant height mode. The Laplacian filtered topographic close-up image of the NPG structure is shown in Figure 4.

Electronic properties

The electronic properties of the 7/13-AGNR and the NPG prepared according to Example 1 were measured.

The band structure calculated by DFT for individual 7/13-AGNRs (left) and the NPG (right) are shown in Figure 5. As can be seen from such figure three types of bands can be found: longitudinal band (L), transversal band (T) and bay/pore band (P). L bands appear confined within the 7-C atom wide backbone of the graphene nanoribbon. On the contrary, T bands are localized within the 13-C atom wide periodic stripes, and thus they do not disperse in the longitudinal direction. They arise from the superlattice periodicity imprinted by the modulated width of the 7/13-AGNR and are therefore exclusive related to its edge topology. The energy gap of the 7/13-AGNR is defined by the L bands which, according to the STS spectra, is of 1.0 eV. This value is significantly smaller than the 1 .5 eV measured for the overall wider 13-AGNR which highlights that not only the nanoribbon width but the edge topology is also relevant for the determination of the gap. L bands remain unperturbed in the NPG, as indicated by the lack of dispersion in the transversal direction (along GC). The DFT band gap is only reduced by 0.12 eV, which agrees with a downshift of similar size measured by STS for the conduction band onset. In contrast, the extension of T band wave functions across the 13-C atom wide section enables substantial inter-ribbon coupling and the formation of 1 D dispersing states with a similar mobility as the longitudinal ones.

Wave functions at G for each of the band examples labelled in Figure 5 are shown in Figure 6. Their dispersion direction is highlighted by guiding stripes.

In Figure 7 (reference A), it can be seen the dl/dV spectra acquired at the multibay edge of a 7/13-AGNR, where the onset of the CB, VB and CB+1 bands can be identified. In the same

Figure 7 (reference B), it can be seen the dl/dV spectra acquired at the peripheral multibay (solid line) and a pore (dashed line) region of an NPG, where the P band is localized. The interaction between the two P states within a pore results in an energy shift of A_bond due to the formation of a bonding band. In A and B references, spectra acquired on Au(1 11 ) is added in shaded color, and the insets show constant height tunneling current (l_t) images (left) and dl/dV maps (right.

As described above in detail, the NPG structure according to this invention has at least one of the following advantages:

The nanoporous graphene structure can work as a semiconductor material; it is suitable for electronic transport;

The nanoporous graphene structure can work as a molecular sieve membrane; it is suitable for sieving ions and molecules;

The nanoporous graphene structure can work as a molecular-sensing membrane; it is suitable for sensing of molecular species by electrical means; - The nanoporous graphene structure has an anisotropic electronic structure with one-dimensional bands for conducting electrons;

The nanoporous graphene structure has atomic precision throughout the structure;

It is a matter of course that the features mentioned above can be used in other combinations in addition to those described without departing from the scope of the invention.

Claims

1. A nanoporous graphene structure having nanopores and nanoribbons, characterized in that comprises monolithically-integrated parallel arrays of nanoribbons and atomically precise nanopores, wherein the nanoribbons are parallel interconnected with identical interconnections between two neighboring nanoribbons to form nanopores with atomic precision and periodic replication between two neighboring interconnections, and the nanoporous graphene structure has one-dimensional conducting channels.

2. Nanoporous graphene structure according to claim 1 , wherein the nanoporous graphene structure has band gaps in the range of 0.1 to 3 eV.

3. Nanoporous graphene structure according to any one of previous claims, wherein the nanoporous graphene structure has extended aromaticity along the longitudinal direction of nanoribbons and non-extended aromaticity along the transversal direction of nanoribbons.

4. Nanoporous graphene structure according to any one of previous claims, wherein the nanopores are monodisperse in size and the nanoribbons are monodisperse in neck width.

5. Nanoporous graphene structure according to any one of previous claims, wherein the periodic replication of nanopores is equal or different along the longitudinal direction of nanoribbons and along the transversal direction of nanoribbons.

6. Nanoporous graphene structure according to any one of previous claims, wherein the nanopore size is < 10 nm², preferably lower than 10 nm².

7. Nanoporous graphene structure according to any one of previous claims, wherein the nanoribbon neck width is < 5 nm, preferably within the range 0.5 - 2.5 nm.

8. Nanoporous graphene structure according to any one of previous claims, wherein the nanoporous graphene structure is obtained from at least one substituted or unsubstituted polycyclic aromatic compound with n aryl groups for inter-ribbon coupling and two or more halide leaving groups (X_a, X_t>, X_c, X_d) for chain polymerization, which has one of the following formulas (l)-(l l):

Formula I

wherein

X_a, Xb, independently, are Br, F, I or Cl;

ni and n₂ are independent integer number from 0 to 10; and n₂ indicate the number of phenyl rings attached to the polyacene backbone, which can be functionalized by Ri,n, R2, R’2, R3,n or R_4,n and/or their C atoms substituted by Si,_n, S_2,n, S3,n or S_4,n. ni and n₂ determine the length of pores; p-i, p_2, p3, p_4, ps, pe, P7, ps, pg, and pio are integer numbers so that pi+p₂ < 9, p3+p₄ < 9, p₅+pe< 9, p₇+ps < 9, p9+pio < 9 ; pi+p₂+1 , p₃+p₄+1 , ps+pe+1 , p₇+ps+1 , P9+P10+ I indicate the number of fused benzene rings in each acene unit that form the polyacene and define the nanoribbon width; ki, k₂, k3, k₄, and k₅ are an integer numbers, so that ki+k₂+k₃+k₄+k₅ < 5; ki+k₂+k₃+k₄+k₅ indicates the number of acenes along the polymerization direction; R₂, R’₂, and R3_,n, are substituents independently selected from a group consisting of hydrogen; a linear or branched or cyclic Ci-C₂o alkyl which is unsubstituted or substituted by one or more OH, halogens, Ci-C₄ alkoxy, Ci-C₄ alkylthio or phenyl; a C₂- C₁₀ alkyl which is interrupted by one or more non-consecutive O; halogens; OR_a; SR_a; CN; NO2; COOR_a; OCOOR_a; OCONR_aR_b; OCOR_a; C1-C12 alkoxy; C1-C12 alkylthio; NR_aR_b; aryl or heteroaryl; wherein R_a and R_b independently of each other, are hydrogen, linear or branched or cyclic C-i-Cs alkyl, aryl or heteroaryl; and Ri_,n and R_4,n are substituents independently selected from a group consisting of hydrogen; a linear or branched or cyclic C₁-C₂₀ alkyl which is unsubstituted or substituted by one or more OH, halogens, C₁-C₄ alkoxy, C₁-C₄ alkylthio; a C₂-C₁₀ alkyl which is interrupted by one or more non-consecutive O; halogens; OR_a; SR_a; CN; NO₂; COOR_a; OCOOR_a; OCONR_aR_b; OCOR_a; C₁-C₁₂ alkoxy; C₁-C₁₂ alkylthio; NR_aR_b; aryl or heteroaryl; wherein R_a and R_b independently of each other, are hydrogen, linear or branched or cyclic Ci-Cs alkyl, aryl or heteroaryl; and

Si_,n, S_2,n, S_3,n and S₄,_n are carbon or heteroatoms independently of each other; Heteroatoms can be nitrogen, phosphor and silicon with the proviso that nitrogen and phosphor heteroatoms do not accept any substituent R attached to it; with the proviso that if is 0, and/or n₂ is 0, then R₂ and/or R’₂, respectively, is/are aryl or heteroaryl; or Formula II

wherein X_a, X_b, X_c, X_d, independently, are Br, F, I, Cl, or H;

ni and n₂ are independent integer number from 0 to 10; and n₂ indicate the number of phenyl rings attached in orto positions to the polyacene backbone that can be functionalized by Ri,_n, R2,n, R3, R 3, R_4,n or R_5,n and/or their C atoms substituted by Si,_n, S_2,n, S3,n, S_4,n or Ss,n. ni and n₂ determine the length of pores; In Formula II ni and n₂ can be 0 since the benzene ring of the benzoacenes can play the role of the linking group; pi, and p_2, are integer numbers so that pi+p₂ < 9, pi+p₂+3 indicate the number of fused benzene rings in each acene unit that form the polyacene; k is an integer number from 0 to 3 ; k+2 indicates the number of acenes along the polymerization direction and defines the nanoribbon width.

Ri,_n, R_2,n, R3, R3, R_4,n and R5,_n are substituents independently selected from a group consisting of hydrogen; a linear or branched or cyclic Ci-C₂o alkyl which is unsubstituted or substituted by one or more OH, halogens, Ci-C₄ alkoxy, Ci-C₄ alkylthio or phenyl; a C₂-Cio alkyl which is interrupted by one or more non-consecutive O; halogens; OR_a; SR_a; CN; N0₂; COOR_a; OCOOR_a; OCONR_aR_b; OCOR_a; Ci-Ci₂ alkoxy; Ci-Ci₂ alkylthio; NR_aR_b; aryl or heteroaryl; wherein R_a and R_b independently of each other, are hydrogen, linear or branched or cyclic C-i-Cs alkyl, aryl or heteroaryl

and

Si_,n, S_2,n, S3_,n S₄,_n and Ss,n are carbon or heteroatoms independently of each other; Heteroatoms can be nitrogen, phosphor and silicon with the proviso that nitrogen and phosphor heteroatoms do not accept any substituent R attached to it.

9. Nanoporous graphene structure according to claim 8, wherein the substituted or unsubstituted polycyclic aromatic compound having n aryl groups for inter-ribbon coupling and two or more halide leaving groups for chain polymerization has one of the following formulas (h, I2, Is, II1, II2):

wherein

X_a and Xb independently are Br, F, I or Cl;

wherein

X_a and Xb independently are Br, F, I or Cl.

wherein

X_a and Xb independently are Br, F, I or Cl.

wherein

X_a, X_b, X_c, X_d, independently, are Br, F, I, Cl, or H;

wherein

X_a, X_b, X_c, X_d, independently, are Br, F, I, Cl, or H.

10. A nanoporous graphene structure according to any one of claims 1 to 9, having one of the following structures (A-F):

1 1. A method for preparing a nanoporous graphene structure according to any one of claims 1 to 10, characterized in that comprises the following steps: a) providing at least one NPG precursor, which has a substituted or unsubstituted polycyclic aromatic compound with n aryl groups and two or more halide leaving groups represented by the formulas (l)-(ll) defined in any one of claims 8 and 9, on a solid substrate;

b) polymerization of the NPG precursors of step a) to form at least one polymer on the surface of the solid substrate;

c) at least partial cylcodehydrogenation of one or more polymers obtained in step b) to convert the polymers into graphene nanoribbons;

wherein the method further comprises:

d) at least partial covalent cross coupling of two or more graphene nanoribbons formed in step c) to laterally interconnect the graphene nanoribbons thereby forming nanoporous in the graphene structure; and

e) optionally, transferring the nanoporous graphene structure obtained in step d) to other solid or liquid environment.

12. Method according to claim 11 , wherein steps b) and c) are induced by thermal activation.

13. Method according to claim 11 , wherein the polymers obtained in step b) and the graphene nanoribbons obtained in step c) are unidirectionaly aligned by means of guiding of a patterned substrate surface.

14. A graphene nanoribbon obtainable by the method defined in any one of claims 11 to 13, wherein the method comprises steps a) to c).

15. A nanoporous graphene structure obtainable by the method defined in any one of claims 1 1 to13.

16. Use of a nanoporous graphene structure according to any one of claims 1 to 10, and 15 for sieving and/or sensing molecular species, and/or for electronic transport.

17. A FET-sensor device comprising a nanoporous graphene structure defined in any one of claims 1 to 10, or 15.

18. A gate-controlled sieve mesh comprising a nanoporous graphene structure defined in any one of claims 1 to 10, or 15.