DK174913B1 - Nanoscale depression formation for substrate surface involves bringing immersed object proximal to surface of substrate by applying two sets of operating parameters - Google Patents

Abstract

An object, partially immersed into a liquid environment, is brought proximal to the surface of a substrate by applying two sets of operating parameters including a bias voltage, a tunnel current and a working potential. The bias voltage of one operating parameter is negative.

Description

DK 174913 B1 Method for Preparing Nanoscale Patterns on a Surface of the Invention

The present invention relates to a novel process for producing nanoholes and nanoscale patterns on solids surfaces at room temperature in a liquid environment using extremely low bias voltages in an electrochemical scanning tunneling microscopy mode. Size-controlled, location-selective and time-stable nanoholes can be routinely formed with this method in a liquid solution without further experimental setups. The feasibility of the formation as well as the stability of nanoscale holes and patterns make them interesting in potential applications in information storage, such as immobilization sites for active biological molecules and otherwise.

15

BACKGROUND OF THE INVENTION

Nanometer and atomic scale modifications of materials have long been desired from both a scientific and a technical point of view. However, direct visualization at the 20 nanometer scale did not become a reality until the first half of this century with the invention of the electron microscope. The advent of local probe microscopes, especially the scanning tunneling microscope (STM), has developed atomic-level imaging and measurement since the 1980s. In addition, STM can serve as a useful tool, not only for imaging with unmatched resolution, but also for controlling and fabricating nanostructures in the nano region.

25

One of the most striking examples is the formation of new functional structures with nanometer or atomic scale on solid surfaces using the STM via its three primary operations: manipulation, removal and deposition.

30 Becker and co-workers (Becker et al., Nature (London) 325 (1987) 419) were first to report on atomic modifications of Ge (111) with the STM. Following this initial result, Eiger et al. many more exciting results using STM.

For example, they deposited single Xe atoms on the Ni (110) surface to form different patterns and constructed quantum circles of single atoms (Eiger et al., Nature (London) 35 344 (1990) 524; Science 254 (1991) 1319; 262 (1993) 218). These results show that individual atoms can be manipulated and desired structures can be built at the atomic level.

These groundbreaking studies were conducted in ultra-high vacuum (UHV) at low temperature (4 K), where atoms were "braced inside" to prevent them from moving around due to their thermal energy.

5

The nanoscale modification of solids at room temperature either in UHV or in air was subsequently explored. Successful examples include nanofabrication of high ~ and / or holes on various materials such as Si, HOPG, metals, semiconductors and superconductors (Lyo et al., Science 253 (1991) 173. Kobayashi et al., Science 259 (1993). 10, 1724. Albrecht et al., Appl. Phys. Lett. 55 (1989) 1727. Lebreton et al., Microelectron.

30 (1996) 391. Sugimura et al., J. Phys. Chem. 98 (1994) 4352. Huang et al., Appl. Phys.

Lett. 61 (1992) 1528. Hosaka et al., J. Vac. Sci. Technol. B13 (1995) 2813. Thompson et al., Nanotechnology 5 (1994) 57). In addition, it has been proven that single molecules can be manipulated with STM spikes at room temperature without degradation of the molecular structure.

Common to these studies is that the suitable nanostructures are implemented at large bias voltages (i.e. bias voltages at several volts), in either constant mode or pulse mode. Extending the working environment to metal-Z liquid solution interfaces would greatly expand the possibility of nanofabrication of surface structures. Thus, a wealth of solutes and ions molecules could be starting materials in controlled adsorption and electrochemical electron transfer mode. Compared to nanoscale modification in UHV, there are few reports of controlled nanofabrication in aqueous, electrochemical environments.

25

Extension of STM to aqueous and other conductive liquids requires operation in electrochemical mode or in situ mode. Control of the separate potentials of the substrate and the STM tip relative to a common reference electrode is of great importance here. Otherwise, the tunnel flow is completely obscured by much larger Faraday flows associated with uncontrolled decomposition of the solvent, metal solution and deposition processes, etc. In addition, insulating material, except the outermost end, must cover the tip. This is because the Faraday currents follow the uncovered electrode region, while the tunnel current is independent of the region and is carried only by a small tip region closest to the substrate surface. Penner et al.

(Penner et al. Appl. Phys. Lett. 60 (1992) 1181) was the first to attempt to apply 35 electrochemical STM to deposit nanoscale metal clusters on HOPG.

3 DK 174913 B1

The preparation of nanoholes on Si (100) in electrochemical environment was also attempted by Ye et al.

(J. Vac. Sci. Technol. B13 (1995) 1423), but the control of the size and placement of holes was not satisfactory and proved to be more troublesome than in air or UHV. Kolb 5 and colleagues (Kolb et al., Chem. Phys. Lett. 209 (1993) 239; Ber. Bunsenges. Phys. Chem.

99 (1995) 1414; Science 275 (1997) 1097. Engelmann, J. Electrochem. 145 (1998) L33) has recently developed a procedure for the local deposition of nanoscale clusters of Cu and Pd on Au (111 surfaces using in situ STM and well-defined electrochemical states. Au (111) - and the peak potentials were initially selected in such a way. that the 10 Au (111) surface was covered with a layer of sub-potential deposited copper and excess copper was also present at the tip, a 50-90 mV potential step temporarily inverting the bias potential, bringing the tip so close to the surface, that a cluster of copper or palladium atoms is transferred from the tip to the surface.

US 4,896,044 discloses a method based on a two-electrode configuration (substrate and tip) and operated in air. The described expansion for operation in liquids does not apply to conductive liquids such as water, where double potentiostatic control and tip coating are important as noted. Topographic holes in the Au (111) surface are implemented when the bias voltage in the constant current mode goes from 0.65 V to a value of at least 2.7 V in 20 500 ps pulses. This increases the tunnel current from 1 nA to values between 10 and 100 nA, which is sufficient to evaporate material from the substrate. There is no evidence of any material around the opening of the craters, but tufts without craters are formed by prolonged exposure to high tunnel currents or bias voltages.

It should be noted that the described expansion according to the method of US 4,896,044 for liquids is not documented and that the nature of the liquids is not specified.

In addition, the disclosed mechanism including metal vaporization in US 4,896,044 is still considered controversial.

It is a disadvantage of the method of US 4,896,044 that it operates only in a non-conductive environment.

It is a further disadvantage of the method of US 4,896,044 that a relatively high bias voltage (at least 2.7 V) must be applied to evaporate material from the substrate, making the process of US 4,896,044 less controllable and therefore less attractive from an industrial point of view.

DK 174913 B1 4

It is an object of the present invention to provide a process for producing nanoholes and nanoscale patterns in a conductive, liquid environment.

It is a further object of the present invention to provide a method in which the manufacture of nanoholes and nanoscale patterns can be appropriately controlled in terms of hole size and nanoscale pattern reproducibility.

BRIEF DESCRIPTION OF THE INVENTION

10

The above-mentioned objects are met by providing a method for forming at least one nanoscale cavity in a surface of a substrate, comprising the steps of: - immersing at least a portion of the surface of the substrate in a liquid environment, - immersing at least a portion of an object in the liquid environment, - placing the object near the surface of the substrate using a first set of operating parameters, which first set of operating parameters including bias voltage, tunnel current and working potential, and - forming of at least one nanoscale cavity in the surface of the substrate using a different set of operating parameters, which second set of operating parameters includes bias voltage, tunnel current and working potential, where the bias voltage of the second set of operating parameters is negative.

The bias voltage is applied between the object and the surface of the substrate. As mentioned above, the bias voltage of the second set of operating parameters is negative. By negative, it is meant that the voltage of the object is lower than the voltage of the substrate surface. The object may form part of a tip of a scanning probe microscope, e.g., a scanning tunnel microscope.

The bias voltage of the second set of operating parameters may be in the range -10 mV - 0V, preferably in the range 8 mV - 0V, advantageously in the range -4 mV - OV, e.g. -2 mV.

The tunnel current of the second set of operating parameters may be in the range 0 -10 nA, preferably in the range 0 - 6 nA, advantageously in the range 0 - 4 nA, and particularly preferably in the range 2 - 3 nA.

The working potential of the second set of operating parameters may be in the range -0.1 - 0.5 V vs 5 NHE, preferably in the range 0 - 0.4 V vs NHE, especially preferably in the range 0.1 - 0.3 V vs NHE.

The bias voltage of the first set of operating parameters can be in the range 100-300 mV, and where the tunnel current of the first set of operating parameters is in the range 0.5-5 nA, and where the working potential of the first set of operating parameters is in the range 0.1 -0.6 v vs NHE.

The liquid environment may be an electrically conductive environment which may comprise an aqueous acidic, basic, neutral or saline electrolyte solution such as HClO 4. The concentration of the HClO 4 solution may be in the range of 10'3-10 M, preferably in the range of 0.02-0.4 M, advantageously in the range of 0.03-0.3 M, and particularly preferably in the range 0.05- Alternatively or additionally, the liquid environment may comprise chloride, sulphate or other adsorbing inorganic or organic anoines or molecules.

Part of the surface of the substrate may contain electrically conductive material such as a metal so that at least one hole is formed in the electrically conductive material. Part of the surface of the substrate may contain a gold film or be bulk gold. Part of the surface of the substrate may contain a semiconductor material such as silicon.

BRIEF DESCRIPTION OF THE DRAWINGS

25

FIG. Figure 1 shows a schematic representation of the difference in STM peak and substrate configuration during formation (a) and imaging (b) of nanoholes.

FIG. 2 shows a standard cyclic voltammogram (CV) of Au (111) in 0.1 M HClO 4 solution. At 30 this figure we can see that Au (111) has an approx. 800-mV double-layer range from 0.1 to 0.7 V vs. NHE (normal hydrogen electrode), the potentiostatic control for nanofabrication was set in this range.

6 DK 174913 B1

FIG. 3 shows in situ STM bi-directional (raw data) of an Au (111) surface consisting of atomically flat terraces in HClO4 solution. Imaging parameters: bias voltage + 200 mV, tunnel current 1.0 nA, scan ranges: (a) 1000 x 1000 nmz and (b) 200 x 200 nm2.

FIG. 4 shows two typical types of nanoholes with triangle shape (a) and ellipse shape (b). Their cross sections are shown in lower sections.

FIG. Figure 5 shows two examples of nanomaterials produced by the method of the present invention: (a) a stable grid of 4 x 4 holes with a 10 grid periodicity of 100 nm and (b) a pattern of "STM" letters. consisting of individual nanoholes with a diameter of about 40 nm.

FIG. Figure 6 shows plots of success rate of nanohole formation in relation to various operating parameters: (a) bias voltage, (b) tunnel current, (c) working potential.

15

FIG. 7 shows the (relative value) dependence of the nanohole volume on various operating parameters: (a) bias voltage, (b) tunnel current, (c) working potential. Each point is an average value of at least 15 individual holes produced under the same operating conditions.

FIG. Figure 8 shows relative volume histograms (Vnsviduai / Naverage) for 87 single nanoholes produced successively using the same operating parameters.

DETAILED DESCRIPTION OF THE INVENTION

25

An HClO4 solution (0.05-0.1 M) was prepared by diluting original, ultra-pure HClO4 (70%) from Fluka with ultra-pure water (Millipore Milli-Q with a resistance of 18.2 ΜΩ), using HClO4 as electrolyte solution in all experiments. All solutions were thoroughly purified with Ar prior to electrochemical STM experiments, and an Ar atmosphere was always maintained during the operations.

The golden cell crystal beads were prepared by melting the ends of Au wire (0.8 or 1.0 mm in diameter, purity> 99.99%) in a hydrogen-oxygen flame (Hamelin, J. Electroanal.

Chem. 407 (1996) 1). The Au (111) facets formed on the single crystal bead were used for STM measurements. The single crystal substrates were cured in the H2 flame and quenched in Millipore water saturated with H2 prior to each experiment. This procedure resulted in atomically flat terraces with a large area that meets the requirements of nanofabrication.

The 5 W tips with a small radius of curvature were prepared by electrochemical etching of W-wire (0.38 mm in diameter) in KOH solution and coated with apiezone wax to reduce or eliminate Faraday currents.

A commercial Rasterscope ™ 3000-EC instrument (DME company, Denmark) equipped with a bipotentiostat for independent substrate and tip potential control was used in both nanofabrication and imaging. Electrochemical control was performed in a specially designed cell compatible with the STM instrument using a three-electrode system.

All work potentials are stated in relation to NHE.

Both nanofabrication and imaging were performed in constant-current mode. The nanofabrication mode is in principle similar to the mode of operation for normal in situ STM imaging. The main points of the present invention are in accordance with the following steps.

A) A relatively large area (typically 1000x1000 nm2) on the substrate surface was selected in advance by STM imaging. This area served as a locus for producing nanoscale patterns consisting of individual holes.

b) Individual nanoholes could be formed in the preselected region (step (a)) of the 25 STM peak scan confined to a suitable scanning area under an extremely small bias voltage (typically -2 mV) and a modest tunnel current (typically 2-3 nA).

c) The desired position of the nanoholes was controlled by adjusting the X and Y axis coordination of the STM tip.

d) The size of holes (10 to 500 nm in diameter) can be controlled by applying STM operating parameters, typically the tunnel current, scan speed, bias voltage, scan range, and working potential.

E) Once the desired individual nanoholes had been prepared by repeating step (b), STM imaging followed normal operating conditions (higher bias voltages) to observe and detect nano patterns.

5 f) Both fabrication and imaging of specific nanomaterials were performed with the same STM tip under the control of the same work potential.

In short, nano-making involves three important steps: 10 1. Pre-selection of an area suitable for nanoscale hole formation 2. Writing process - preparation or formation of nanoscale holes 3. Registration process - imaging of the nanoscale holes produced

Regarding the writing process, typical operating parameters were the bias voltage 15 of -2 mV, the tunnel current of 2-3 nA and the working potential at 0.3 V vs NHE for the writing.

A bias voltage of 200 mV, a tunnel current of 1.0-1.2 nA and working potential at 0.3 V vs NHE were typical parameters during the pre-selection and registration process.

20

Figure 1 schematically shows the difference in distance and state between the STM tip and the substrate during formation (Figure 1 (a)) and the imaging (Figure 1 (b)) of nanoholes. This difference can be realized by changing the bias voltage and tunnel current. When the distance between tip and substrate is reduced to a threshold by applying an extremely small bias voltage, the tip's interaction with the substrate becomes strong enough to form nanoholes on the substrate surface. It is also necessary to hold the STM tip with negative polarity vs substrate. These are basic principles of nano-hole formation. In contrast, subsequent imaging takes place at greater distances and smaller interaction between the tip and the substrate by presetting a higher bias voltage.

30

A standard cyclic voltammogram (CV) of Au (111) in HClO * is shown in FIG. 2. There is a wide double-layer voltage range (about 800 mV) from -0.1 to 0.7 V vs. NHE, which range is limited by dihydrogen evolution at low voltages and formation of several gold oxide phases at high voltages. These voltammetric properties are consistent in all respects with reported patterns of Au (111) in this solution. The potentiostatic control was set within this range for writing and recording processes to avoid the involvement of any unnecessary electrochemical reactions.

Figure 3 shows typical in situ STM images of the Au (111) surface consisting of atomically flat 5 terraces over several hundred nanometers. Single atomic roof terraces are visible in FIG. 3 (b).

Thus, the formation of nanoholes is easily distinguished from the background of these practically flawless surfaces. Such images were always recorded before making nanoscale patterns.

They served as the preselected area for the placement of specific nanomaterials.

FIG. 4 shows in situ STM images of two individual holes. They represent two typical forms of holes often formed in the present preparation. The lateral distribution is approx. 40 nm, and the depth approx. two atomic layers. Single atomic layer structures can be seen in both images and the details of the hole structures are further demonstrated in their cross section. The FIG. 4 (a), the triangle shown in the lateral form is optionally joined to Au (111) own surface structure, while the hole in FIG. 4 (b) is more elliptically shaped. Substrate atomic highs can be seen close to the surrounding edge of the holes, which corresponds to the voltage range with full electrode polarization and no gold electrochemical solution. The holes are stable for days, and during the following manufacture and imaging there was no sign of degradation.

This forms the basis for the preparation of patterns consisting of nanoholes, two examples of which are shown in FIG. 5th

The FIG. 5 (a) is a stable grid consisting of 4 x 4 holes with a lattice periodicity of 100 nm, whereas in FIG. 5 (b) is the three letters "S T M". Holes accompanied by highs or tufts at the edges are consistently seen.

25 The feasibility of such pattern formation and their stability make them technologically interesting in the context of information storage, as immobilization sites for active enzymes and in other ways as briefly described below.

Operating conditions affecting the formation and size of nanoholes have been systematically investigated. FIG. Figure 6 shows how the probability of hole formation depends on the tunnel current, bias voltage and working potentials. Hole formation is possible in most of the bilayer stress region of the Au (111) substrate with over 90% efficiency in the range 0.1-0.3 (Fig. 6 (c)). A threshold effect is associated with the phenomenon as seen in Fig. 6 (b). In addition, the efficiency is close to 100% for tunnel currents lt> 1 nA up to 10 nA, which is 35 the highest possible value of the commercial Rasterscope ™ 3000-EC instrument. Under lt 10 DK 174913 B1 «1 nA, efficiency drops rapidly and approaches zero when lt -» 0.5 nA. FIG. 6 (a) shows the most interesting feature of the hole formation process. In the electrode voltage and tunnel current areas with maximum efficiency, ie. 0.1 V-0.3 Vog 1 nA-10 nA, respectively, nanoholes are formed only in an extremely narrow and low bias voltage range, ie.

5 literally a few mV. This "jump" property is almost similar to critical behavior. The sharp maximum is also very close to zero bias voltage, but consistent with small negative substrate peak bias voltages. The bias voltage-dependent characteristic is very different from all other reported nano-producing cases. in either in situ or ex situ STM modes.

10

FIG. Figure 7 is an overview of the normalized individual hole volume (each point is based on an average value of at least 15 individual holes made under the same conditions) as the external parameters, bias voltage, tunnel current, and substrate electrode voltage are varied. FIG. 7 (a) again shows the obvious criticality;

7 (b) shows the threshold effect, while FIG. 7 (c) shows the weak dependence on the substrate electrode voltage in the electrochemical bilayer region of Au (111) in aqueous HClO 4.

The distribution of individual hole sizes can be evaluated from FIG. 8. This figure is based on 20 statistics of 87 individual holes that were produced continuously under the same operating conditions. The profile approximates a Gaussian distribution.

Physical mechanisms in prior processes for STM-induced nanostructure formation in vacuum or air include: (a) Pulse- and voltage-induced local metal ion evaporation or melting. These effects require high fields, ie. 107 V cm'2 or higher.

(b) Field gradient-induced surface diffusion of metal atoms. This, in turn, is caused by high voltage pulses that heat the tip and transfer atoms or adatoms to the substrate surface.

(c) Electrostatic forces that induce elastic deformation in the substrate and tip as the tip comes closest to the surface. At sufficiently strong fields, the tip contacts the surface, with possible formation of nanoscale indentations.

11 DK 174913 B1 (d) Electron-phonon interactions (the Nottingham effect) which eventually also lead to local melting.

5 (e) Field-assisted diffusion and slip. The former is based on high electric field inhomogeneity in the gap region, the latter on a combination of non-covalent chemical interactions between tip and surface atoms.

These effects all appear to contrast with the present invention, where extremely small bias voltages induce hole formation. An approach to solving this problem is not straightforward, but the following remarks regarding the spontaneous reconstruction of precious metal surfaces in electrochemical environments and narrow charge regions are in place.

The charge flow to the substrate surface is always potentiostatically controlled by the external circuit of electrochemical STM. External charge flow is also what ultimately induces surface reconstructions and other surface phase changes. However, the tip in in situ STM mode has a finite charge and represents a finite local field at the substrate surface when the tip is very close to the surface.

20 There are methods for calculating such a field. Theoretical approaches to surface reconstructions of precious metals in electrochemical environments have also been implemented, with several concepts suitable for the present invention. Preferably, they are: a) Surface charge-induced phase changes of the metal surface, b) Phase diagrams including ordered, non-ordered and rough c) Observations that phase changes between the ordered phases continue through unordered and rough phases, and 30 d) Observation that these changes are continuous.

As a suggestion, the observed, obvious, critical bias voltage-dependent behavior and the threshold effect of the preset tunnel current in the nanohole annealing process could reflect a continuous phase change induced by the field of the peak. Such an effect could rest on small physical interactions, in contrast to the large stress effects otherwise required for formation of metal surface nanostructures in vacuum or air - in, for example, US 4,896,044. One such, currently presumed, scenario involves possibilities for further investigation, including in particular the effects of anion adsorption and the effects of the charge and the possibility of zero charge in the tip materials.

The 5 nanoholes were obtained for Au (111) with W-wire as a point source, but the method is not limited to these materials.

10 Metal nanoscale structures should incorporate a number of technological perspectives. The value of the above described methodology for nanohole structure fabrication is also promising as structural elements of future nanotechnology. The following options are mentioned here. In a first embodiment, nanohole arrays and pattern formation at preselected locations comprise 15 tools for information storage, writing and reading processes, and nanoscale lithography. For example, holes can also be combined to form grooves or other structures.

In another embodiment, the holes or hole arrays can be used as nucleation sites for depositing metals other than the substrate metal. This involves nano- and 20 microelectronic perspectives for metal combination, material doping, comprehensive construction of nanoscale devices, etc. The holes can serve as templates for deposited metal, semiconductor, semiconductor and insulator deposition in a wide range of functional, organized composite materials. If the procedure is applied to thin layers of Au or other metals deposited on other conductive or semiconducting materials, then 25 holes can be made through the gold layers. This would be important in sub-micro meter reading devices.

In a third embodiment, the nanoholes are potential adsorption or encapsulation sites for molecular or biomacromolecule immobilization. This includes basic and 30 technological perspectives on nanoscale molecular and biomolecular dynamics.

In a fourth embodiment, the holes can be made to function as nanoscale chemical "reactors" where chemical processes at the molecular or mesoscopic level can be controlled.

In a fifth embodiment, metalloenzymes could be immobilized in the gaps of preferential adsorption or covalent immobilization. Given the very gentle conditions of controlled hole formation, sequential hole formation and enzyme immobilization could provide a basis for organized arrays of various functional enzymes. In this way, 5 versatile, multifunctional enzyme electrodes specific for several different substrate molecules in the solution could be constructed.

In a sixth embodiment, hole formation and pattern organization are affected and may possibly be controlled by specific anion adsorption, as anion adsorption is known to greatly influence both the reconstruction stress and the intermediate surface phases. Specific patterns of these properties can also be expected to be transferred to other precious metal electrode surfaces and surface crystal planes.

Claims (14)

A method of forming at least one nanoscale hole in a surface of a substrate, comprising the steps of: 5 - immersing at least part of the surface of the substrate in a liquid environment, - immersing at least part of a substrate positioning the object near the surface of the substrate using a first set of operating parameters, and - forming at least one nanoscale hole in the surface of the substrate using a second set of operating parameters, characterized in that the floating environment is electrically conductive and knows that the first and second set of operating parameters include bias voltage, tunnel current and working potential, where the biass voltage of the second set of operating parameters is negative. 20 Method according to claim 1, characterized in that the object forms part of a scanning probe microscope, for example a scanning tunnel microscope. Method according to claim 2, characterized in that the object forms part of a tunnel tip. Method according to any one of the preceding claims, characterized in that the bias voltage of the second set of operating parameters is in the range -10 mV - 0 V, preferably in the range -8 mV - 0 V, advantageously in the interval - 4mV - 0 V, e.g. -2 mV. 30 Method according to any of the preceding claims, characterized in that the tunnel current of the second set of operating parameters is in the range 0-10 nA, preferably in the range 0-6 nA, advantageously in the range 0-4 nA, and in particular preferably in the range 2 - 3 nA. 35 Method according to any one of the preceding claims, characterized in that the working potential of the second set of operating parameters is in the range -0.1 - 0.5 V vs NHE, preferably in the range 0 - 0.4 V vs NHE, advantageously in the range of 0.1 - 0.3 V vs NHE. in Method according to any one of the preceding claims, characterized in that the bias voltage of the first set of operating parameters is in the range 100 - 300 mV and wherein the tunnel current of the first set of operating parameters is in the range 0.5 - 5 nA. , and where the working potential of the first set of operating parameters is in the range 0.1 - 0.6 V vs NHE. DK 174913 B1 2 Process according to claim 1, characterized in that the electrically conductive environment comprises an aqueous acidic, basic, neutral or saline electrolyte solution such as HCICV Process according to claim 8, characterized in that the concentration of the HClO-5 solution is in the range 10'3 - 10 M, preferably in the range 0.02 - 0.4 M, advantageously in the range 0.03 - 0. 3 M and especially preferred in the range 0.05 - 0.1 M. Method according to any one of the preceding claims, characterized in that part of the surface of the substrate contains an electrically conductive material such as metal, such that the hole, of which there is at least one, is formed in the electrically conductive material. Method according to any one of the preceding claims, characterized in that part of the surface of the substrate contains a gold film or constitutes bulk gold. Process according to claim 1, characterized in that the liquid environment comprises chloride, sulphate or other adsorbing, inorganic or organic anions or molecules. Process according to any one of the preceding claims, characterized in that part of the surface of the substrate contains a semiconductor material such as silicon. 20 Method according to claim 1, characterized in that the object forms part of a tip of a commercially available scanning probe microscope, for example a scanning tunnel microscope.