WO2018135983A1 - Switchable plasmonic display device - Google Patents

Abstract

A reflective display device comprising a conductive polymer layer in communication with an adjacent plasmonic structure adapted to reflect a selected colour from incident light, said plasmonic structure comprising: a first metallic layer provided on top of a spacer layer, and a visible light reflective layer provided below the spacer layer, wherein the plasmonic structure is provided on a substrate.

Description

SWITCHABLE PLASMONIC DISPLAY DEVICE

FIELD

The present invention pertains to the field of display devices and more particularly to reflective display devices comprising plasmonic structures.

BACKGROUND

A reflective display has low power consumption in comparison with active displays such as those based on light emitting diodes. Often these reflective devices, commonly referred to as "electronic paper" only require power when switching on/off visibility of an individual region.

Reflective (paper like) displays typically use electrophoretic ink. However, these technologies are limited to simple, generally monochromatic implementations such as those found in e-Readers such as the Amazon Kindle®.

A display device which can selectively display colours throughout the visible light spectrum (ca. 390nm - 700nm), and not just black and white would clearly be desirable.

Plasmonic metasurfaces are a class of subwavelength architectures that have been considered to show many benefits in comparison to traditional pigment based ink printing. The structural colour arising from nanostructured metallic surfaces may have benefits such as increased resolution and scalability of optical responses with structural dimensions. (Franklin, D. et al. Polarization-independent actively tunable colour generation on imprinted plasmonic surfaces. Nat. Commun. 6: 7337) However, to date these advantages have mainly been considered in terms of static assemblies and few advances have been made with respect to dynamic displays taking advantage of plasmoni c metasurfaces. That is, there has to date been no implementations that are manufacturable at a large scale and capable of producing dynamic displays in which the visibility/reflectivity of regions of the plasmonic metasurfaces can be modulated.

A device which is flexible, that is, non-rigid is also desirable as such devices can be used in cases where traditional rigid display devices are not-suitable or prone to damage.

In light of the above it would be desirable to have a flexible low-power multi-colour display.

SUMMARY

Accordingly, the present invention preferably seeks to mitigate, alleviate or eliminate one or more of the above-identified deficiencies in the art and disadvantages singly or in any combination and solves at least the above mentioned problems by providing a reflective display device comprising a conductive polymer layer in communication with an adjacent plasmomc structure adapted to reflect a selected colour from incident tight. Said plasmonic structure comprising: a first metallic layer provided on top of a spacer layer, and a visible light reflective layer provided below the spacer layer. The plasmonic structure is provided on a substrate.

A display assembly comprising a plurality of devices is also provided.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects, features and advantages of which the invention is capable, will be apparent and elucidated from the following description of embodiments of the present invention, reference being made to the accompanying drawings, in which:

FIG. 1 is a schematic view of a flexible display device according to an embodiment of the invention.

FIG. 2a,b is a schematic view of a flexible display device according to an embodiment of the invention having a plurality of plasmonic structures.

FIG. 3 is a schematic view of a flexible display device showing the modulation of conductive polymer transparency where FIG. 3 a shows the conductive polymer having a low transparency and FIG. 3 b shows the polymer having an increased transparency and therein exposing the plasmonic structure.

FIG. 4 shows the reflectivity spectra for devices reflecting red, green and blue. Where

Ron is reflecting red and where the conductive polymer is transmitting light, or in a switched on state. Roff is reflecting red and where the conductive polymer is in a switched off state. Bon, Boff, Gon, Goff are similar but for devices adapted to display blue and green colours.

FIG. 5 shows power density of devices for displaying blue, green and red compared with average power use for black and white electrophoretic displays and active-matrix organic light-emitting diode (AM-OLED) displays.

FIG. 6 shows switching dynamics (for a red device) showing current (electrode area 176 mm2) and reflectivity at 660 nm versus time as the voltage is reversed (>45% of the reflectivity change occurs in <0.8 s).

FIG. 7 shows reflectivity measured with two different objectives that illuminate and collect light from different angular intervals (NA 0.25 or 0.65 in air). FIG, 8a-d shows specular reflection from the plasmonic structures in an at high angles of incidence. FIG 8e shows a green control sample with no nanoholes, which leads to a different specular reflectivity and another colour.

FIG 9 shows an example of the current upon electropolymerization. The electrode area was 112mm2. At time zero a voltage of +0.57 V (vs Ag/AgCl) was applied to grow the polymer film.

FIG. lOa-b shows extinction changes upon electropolymerization (as in Figure 9) of "ordinary" nanohole arrays, identical to those on the metasurfaces but prepared on glass. A schematic of the structure and the polymerization is also shown.

FIG. 10c shows Extinction spectrum of ordinary nanohole arrays before and after electropolymerization of -200 nm polymer as well as at different potentials (with polymer). The peak at 720 nm represents coupling to surface plasmons.

FIG. 11a shows extinction changes at one wavelength as the voltage is slowly changed. The marked regions show that the transparency saturates before the voltage reaches its minimum. The polymer thickness is here ~150 nm (dry).

FIG. 1 lb shows Extinction changes at three different wavelengths for polymer- functionalized

ordinary nanohole arrays when the voltage is switched between +0.3 V and -1.0 V. The dry thickness of polymer was 320 nm in this case.

FIG. 12 shows response times determined from data present in Figure l ib. (The response time was defined as the time to reach 90% of the extinction change.) The dashed lines indicate the polymer thickness used for modulating each colour for the devices.

FIG. 13 is a representative voltammogram (3 cycles) with scan rate 10 mVs^-1. The electrode area was 176 mm2. The lighter dashed line shows the voltammogram after performing 10000 switching cycles between +0.3 V and -1.0 V.

FIG. 14 shows a Current trace when switching the voltage of a green sample with 230 nm polymer. The electrode area was 176 mm2. The charge transfer for a positive pulse is equal in magnitude to that of a negative pulse. The current during one negative pulse is magnified and the background and average currents are indicated.

FIG. 15 shows coloration memory defined as the time until the optical absorption of the polymer returns to 90% of its maximum after releasing the negative potential. The values are about one order of magnitude higher than the response time for switching to +0.3 V FIG, 16 shows dispersion relation for one of the surface plasmon modes in a thin film multilayer corresponding to the blue plasm onic metasurfaces. The field magnitude is also shown for the transverse magnetic mode at a wavelength of 570 nm.

FIG. 17 shows convoluted reflectivity from RGB pixel combinations generating secondary colours.

DETAILED DESCRIPTION

Embodiments of the present invention will be described in more detail below with reference to the accompanying drawings in order for those skilled in the art to be able to carry out the invention. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

The following detailed description describes a reflective display device 1 adapted to reflect and absorb incident light and therein display a colour through the use of a plasmonic structure 101 and a conductive polymer layer 100. The transmission of incident light to the plasmonic structure 101 can be modulated by the switching of the redox state of a conductive polymer layer 100 provided adjacent to the plasmonic structure 101. By modulating the transmission of incident light the visibility of the plasmonic structure can be modulated leading to a dynamic controllable display device. The device displays outstanding colour intensity, and due to the thin film nature of the plasmonic structures 101 and the conductive polymer layer 100 can be provided on a substrate 200 that is flexible, that is, non-rigid.

FIG. 1 shows the reflective display device 1 comprising a conductive polymer layer 100 in communication with a plasmonic structure 101 comprising a plurality of vi sible light reflecting/absorbing layers deposited on a substrate 200. A voltage may be applied across the conductive polymer layer 100 such that absorption and/or scattering of visible light by the conductive polymer layer 100 is decreased (and therefore the transmission of incident light to the plasmonic structure 101, through the conductive polymer layer 100 is increased). The plasmonic structure 101 comprises a first metallic layer 102, a spacer layer 103 and a visible light reflective layer 104. Incident light L to the device which is transmitted through the conductive polymer layer 100 is substantially reflected, scattered or absorbed by the plasmonic structure 101. That is, it is not transmitted through the device I .

The first metallic layer 102 may be provided adjacent to the spacer layer 103. The spacer layer 103 may be adjacent to the visible light reflective layer 104. Each layer may be in contact, and in communication, with its adjacent layer. The first metallic layer 102 may be provided above, such as on top of, the spacer layer 103, The spacer layer 103 may be provided above, such as on top of, the visible light reflective layer 104. The order of layers herein described is intended to be understood with the uppermost layers first receiving incident light, i.e. incident light L strikes the top of the device. An order of layers of the device 1 is shown in FIG. 1.

The plasmonic structure 101 is designed such that it absorbs/reflects light at selected wavelengths. The metasurface feature of plasmonic structures is known in the art. For example, the plasmonic structure 101 may be designed such that it is capable of absorbing light in the spectrum of light corresponding to the visible blue spectrum. Incident light striking the surface of the plasmonic structure is thus both partially reflected, scattered and absorbed. In the above example citing visible blue light, the light is absorbed predominantly in the spectrum of wavelengths corresponding to visible blue light. In such an example, light in the wavelengths not corresponding to the blue pari of the visible light spectrum is substantially reflected. This results in the plasmonic structure, on the application of white light incident to the surface of the plasmonic structure 101, appearing to be red coloured to an observer.

One technique of achieving the functionality of different reflected colours is to alter the arrangement and thickness of the plurality of layers of the plasmonic structure 101. This will be further described below.

The visible light reflective layer 104 may be deposited on the substrate 200. The visible light reflective layer 104 acts as a mirror layer. The visible light reflective layer 104 reflects light which has not been reflected, scattered or absorbed by the conductive polymer layer 100, the first metallic layer 102, and the spacer layer 103, or any other layers which may be present in the device 1 (such as thin film adhesion layers) and positioned between an incident light L source and the visible light reflective layer 104. The visible light reflective layer 104 may be a thin film mirror layer. The thin film mirror layer 104 deposited on the substrate 200 has the advantage that the thin film is flexible and highly reflective. The thin film mirror layer 104 may comprise, such as be, a thin film layer of silver (Ag). A silver layer is highly reflective in the visible spectrum and therein provides a high base reflectivity for the flexible display device 1. The thin film mirror layer may be a thin film silver layer having a thickness in the range 20nm - 250nm, such as 50 nrn - 200 nm, 1 O0nm - 200nm, or as is shown in the following experimental section, around 150nm. A silver layer having a thickness of greater than about 80nm provides a very high base reflectivity in the visible spectrum. Other highly reflective materials may be selected for the visible light reflective layer 104.

The first metallic layer 102 of the plasmonic structure 101 may be a thin film metallic layer. The first metallic layer 102 of the plasmonic structure 101 may comprise, such as be, a gold (Au) layer. The first metallic layer 102 may be a layer having a thickness in the range of 10nm - 50nm, such as 1 5-25nm, or about 20nm, The first metallic layer 102 may be provided with an array of nanostructures 120. The metallic nanostructures 120 may be an array of substantially circular nanoholes. The nanostructures 120 provided in the first metallic layer 102 enhances the coloration since it enables coupling to surface plasmons and provides strong resonant scattering. A plasmonic structure 101 , comprising a first metallic layer not comprising nanostructures 120, is capable of reflecting selected colours, however, such a layer does not support plasmon excitation under ordinary illumination and cannot scatter light. A layer comprising nanostructures 120 displays improved scattering of complementary colours, that is, those colours not absorbed by the plasmonic structure, and therein displays improved colouration.

The nanostructures may be a plurality of nanoholes having a diameter in the range lO0nni - 200nm. Such as, 120nm - 180nm, 155nm - 165nm or about 158nm. The plurality of nanoholes may be an array of nanoholes arranged in a matrix, such as a grid. The nanoholes may be provided at regular intervals or in some cases may be spaced irregularly. The mean value of the centre-to-centre distance between adjacent nanoholes may be between 250nm - 350nm, such as 300nm - 320nm. The nanoholes may be prepared via colloidal self-assembly and subsequent tape stripping.

Depending on the colour to be reflected and the metal selected for the first metallic layer 102, the layer 102 need not comprise nanostructures. For example, if the first metallic layer 102 comprises copper (Cu) then nanostructures 101 may not be necessary for plasmonic structures 101 adapted for reflecting red light.

The spacer layer 103 may be a dielectric spacer layer 103. The spacer layer 103 may comprise, such as be, a transparent dielectric material, such as aluminium oxide or titanium oxide. A suitable dielectric is aluminium oxide (AI2O3) also known as alumina. The provision of the spacer layer tunes the reflective colour via Fabry -Perot interference.

The thickness of the spacer layer 103 may be between 30nm - lO0nm, such as 40nm - 95nm. The inventors have found that to achieve reflectivity of a selected colour in the plasmonic structure 101, a specific thickness of the spacer layer 103 is required. In

combination with the first metallic layer 102, and the visible light reflective layer 104, the inventors have identified that an alumina dielectric spacer 103 having a thickness of about 43 - 53 nm, or about 48nm corresponds to a highly reflective primary red colour. An alumina dielectric spacer having a thickness of 88 - 98 nm, or about 93nm corresponds to a highly reflective primary green colour. An alumina dielectric spacer having a thickness of 78 - 88 nm, or about 83nm corresponds to a highly reflective primary blue colour.

As is apparent from the description above, a plasmonic structure reflecting a single primary colour may have a dielectric spacer layer 103 selected from one of about 48nm (red), 93 nm (green), or 83nm (blue). A device 1 according to the invention reflecting more than one of the primary colours requires more than one spacer layer 103, wherein each of the more than one spacer layers 103 is provided with a different thickness, such as two, three or a plurality of thicknesses wherein each thickness corresponds to a desired colour.

The inventors have found that through the provision of a conductive polymer layer 100 adjacent to the plasmonic structure 101 the visibility of the plasmonic structure 101 to incident light can be modulated and thus a dynamic, flexible, multi-colour display is achieved. Light incident to the display device 1 may be either substantially absorbed and/or scattered, or transmitted by the conductive polymer layer 100.

The conductive polymer layer 100 comprises a polymer having a transparency which can be modulated through the application of an electric potential across the conductive polymer layer. The conductive polymer layer 100 can comprise, such as be, polypyrrole (PPy). As is shown in FIG. 3a and b polypyrrole is substantially opaque without the application of a potential across the conductive polymer layer 100. On the application of a potential across the conductive polymer layer 100, the layer 100 has an increased transparency, reduced opacity and therein exposes the underlying plasmonic structure 101 to incident light L. An alternative conductive polymer may be polyaniline or a polymer based on a 3,4-ethylendioxythiophene monomer such as poly(3,4-ethylenedioxythiophene) (PEDOT) or poly(3,4- ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS). The conductive polymer layer 100 may be prepared by electrochemical polymerization or by chemical deposition on the plasmonic structure 101.

For improved colour reflectivity performance, the thickness of the conductive polymer layer 100 may be adjusted with respect to the colour of the underlying plasmonic structure 101. if the plasmonic structure 101 is to reflect a primary red colour then the thickness of the e.g., polypyrrole layer 100 should be in the range IS0nm - 250nm, such as 180nm - 200nm or about 190nm. if the plasmonic structure 101 is to reflect a primary green colour then the thickness of the e.g., polypyrrole layer should be in the range 200nm - 300nm, such as 250nm - 270nm, or about 260nm. if the plasmonic structure 101 is to reflect a primary blue colour then the thickness of the polypyrrole layer should be in the range 50nm - 1 50nm, such as lO0nm - 120nm, or about 1 l 0nm. If the plasmonic structure 101 is to reflect another colour than red, green, blue then the thickness of the polypyrrole layer should be adjusted accordingly to achieve suitable contrast between the on and off states for the wavelength corresponding to the desired colour.

As can be seen in FIG. 4 a significant advantage of the current device 1 is that the peak wavelength of the reflected light is not substantially shifted on switching/modulating the redox state of the conductive polymer layer 100. The peak wavelength for at least blue and green wavelengths is shifted by less than about 50 nm.

To achieve increased transparency an electric potential must be applied across the conductive polymer layer 100. The potential/voltage applied across the conductive polymer 100 may be in the range -0.7V to -1.1 V. The ideal voltage was found to be dependent on the thickness of the conductive polymer layer 100 and/or the colour to be reflected by the plasmonic structure 101. For a plasmonic structure 101 and conductive polymer 100 assembly for reflecting/displaying a red colour the ideal voltage was found to be about -0.9V For a plasmonic structure 101 and conductive polymer 100 assembly for reflecting/displaying a green colour the ideal voltage was found to be about -1.0 V. For a plasmonic structure 101 and conductive polymer 100 assembly for reflecting/displaying a blue colour the ideal voltage was found to be about -0.8V.

To achieve a potential across the conductive polymer layer 100 the potential is applied between the first metallic 102 layer of the plasmonic structure 101 and a reference electrode (not shown). A conductive medium 300 is provided adjacent an in electrical connection to the conductive polymer layer 100 and to the reference electrode. The conductive medium 300 may for example be a conductive solution comprising an electrolyte. A typical conductive solution is a solution comprising at least one alkali metal such as a solution comprising sodium chloride or lithium chloride.

Between each of the first metallic layer 102, spacer layer 103 and visible light reflective layer 104 a thin film adhesion layer 105 may be provided. The adhesion layer 105 is provided to increase adhesion of a layer (102, 103, 104) to its respective adjacent layer. The adhesion layer may comprise for example Chromium (Cr).

The substrate 200 may form a support for the piasmonic structure 101 and the conductive polymer layer 101 and optionally, the conductive medium 300. As described above, as neither the conductive polymer layer 100 nor the piasmonic structure 101 are fragile, whilst also being very thin, both may be flexible, a flexible substrate 200 may be used. The flexible substrate may comprise a polymer such as a plastic. An example of a suitable polymer is polyethylene terephthalate, or PET. The substrate 200 is substantially planar. The top surface of the substrate 201, that is, the surface adjacent to the piasmonic structure 101 is ideally flat and plane. That is, the top surface 201 is provided without surface features such as gratings. This allows the piasmonic structure 101 to be deposited on a flat substrate. The substrate 200 may be rigid, however, then the device 1 is also rigid.

The spacer layer 103, and the visible light reflective layer 104 may also be substantially planar. That is, the layers do not comprise structures such as gratings which are complex to manufacture.

An additional oxidation prevention layer may be provided between the first metallic layer 102 and the conductive polymer 100. This layer may prevent oxidation of the first metallic layer 102. A suitable material for the oxidation prevention layer is AI2O3. The layer may have a thickness of from about 10 to 50 nm.

FIG. 2 shows a display device 2 capable of displaying a plurality of colours, such as the primary colours red, green and blue. As described above, via modification of the thickness of the spacer layer 103, the display device is also capable of displaying other, non-primary colours.

The display device 2 of FIG. 2 comprises a first conductive polymer layer 100, a plurality of piasmonic structures 101 for reflecting a colour as described above, provided on a substrate 200, wherein each of the plurality piasmonic structures 101 is adapted to reflect a colour. Each of the plurality of piasmonic structures 101 has a spacer layer 103 (not shown in FIG. 2a) having a thickness dependent on the colour to be reflected. As shown in Figure 2b The visible light reflective layer 104 of the piasmonic structures may be provided covering the entire substrate 200, or a portion thereof, that is the visible light reflective layer 104 may be shared by multiple adjacent piasmonic structures 101 . The device may further comprise a plurality of conductive polymer layers 100 wherein each of the conductive polymer layers 100, adjacent to each of the plasmonic structures 101, may also have a thickness dependent on the colour to be reflected.

In the device 2 shown in FIG. 2, the conductive polymer layer 100 has a uniform thickness. This simplifies the manufacturing process but leads to reduced performance, as, ideally, as described above, the conductive polymer layer 100 has a thickness adjusted dependent on the colour to be reflected by the plasmonic structure 101.

Also provided is a flexible display assembly comprising a plurality of devices 1,2, wherein each device 1,2 is designed to reflect at least one selected wavelength/colour and has a conductive polymer layer 100 being switchabie between a first substantially opaque (non- visible light transmitting) mode, and a second substantially transparent (visible light transmitting) mode. Each of the devices 1 ,2 for reflecting at least one selected wavelength spectrum/colour comprises at least one plasmonic structure 101 according to that described herein. The first metallic layer 102, of each of the plasmonic structures 101 may be electrically isolated from the first metallic layer 102 of its adjacent plasmonic structure 101. In this way the transmission of incident light through the conductive polymer layer 100 may be modulated for each device 1, or each plasmonic structure 101. The plasmonic structures 101 may be arranged in a grid as shown in FIG. 2.

The flexible display assembly may comprise a plurality of pixels, wherein each pixel is a triplet of display devices 1 arranged adjacent to one another and wherein each of the first, second and third devices has a plasmonic structure 101 arranged for reflecting a unique colour. That is, the colours are not repeated within the triplet. The colours may be, for example, red, blue or green, however they need not be limited to these primary colours. Each pixel may have a width/length of at least about 5 μιη . The pixel size is selected such that each corresponds to the resolution of the human eye taking in to account the expected distance of a human observer to the display device, larger pixel dimensions are generally easier to fabricate whilst reduced dimensions result in an increase in potential resolution in a display assembly. Each pixel may be separated by at least about 1-10 μιη.

The display assembly may be provided with a conductive medium 300 adjacent to the conductive polymer layer(s) 100. The conductive medium 300 may be a fluid such as a conductive solution comprising an electrolyte. The conductive medium 300 is in electrical connection to the conductive polymer layer(s) 100. The conductive medium may be a planar, substantially solid conductive medium 300 provided in electrical connection with each of the plurality of devices 1 of the assembly. Each of the plurality of devices I may be provided with an electrically connection to a controller such that a potential can be applied to the conductive polymer layer 100 of each device 1 through the plasmonic structure.

As described above, the substrate 200 may be flexible, resulting in a flexible display- assembly 2.

Incident light L to the device lor assembly 2 may be white light, comprising a combination of wavelengths in the visible spectrum. The incident light may be sunlight. Also provided herein is the use of a device 1 or assembly 2 as described above for the dynamic display of at least one colour wherein incident light provided to the device 1 or assembly 2 is sunlight.

Manufacturing methods are disclosed in the fabrication section.

The term visibility has been used above to describe whether light incident to the device is reflected from the device to an observer. As is apparent to a person skilled in the art the term visibility of a specific colour implies that a portion of the incident light is absorbed or scattered and a portion is reflected. No transmission occurs through the device 1 as a whole. Furthermore, whilst colours have been referred to in the singular it is apparent that a colour to be reflected/ displayed by the device comprises a spectrum of wavelengths with a peak corresponding to that colour. With respect to blue, green and red, these have peaks at approximately 475 nm for blue, 510 nm for green and 650 nm for red.

While the invention has been described in conjunction with a number of embodiments, it is evident that many alternatives, modifications and variations would be or are apparent to those of ordinary skill in the applicable arts. Accordingly, Applicant intends to embrace all such alternatives, modifications, equivalents and variations that are within the scope of the invention.

Fabrication

Layer structure

Flexible Poly(ethylene terephthalate) (PET) substrates 200 were cleaned by sonication in ethanol for 5 min. Subsequently, a first adhesion layer (5nm - Cr), followed by the visible light reflective layer 104 (150nm - Ag), were deposited on the substrate 200.

The dielectric spacer layer 103 (varying thickness - A1203) was subsequently deposited on the surface of the visible light reflective layer 104. Colloidal lithography was performed on freshly deposited AI2O3 as described in, for example, J. Junesch, T. Sannomiya, A. B. Dahlia, ACS Nano 2012, 6, 10405; A, B, Dahlin, M, Mapar, K. L, Xiong, F. Mazzotta, F, Hook, T. Sannomiya, Adv. Opt. Mater. 2014, 2, 556; A. B. Dahlin, R. Zahn, J. Voros, Nanoscale 2012, 4, 2339, using a batch of 158 ± 4 nm colloids (Microparticles GmbH), which gave a short- range ordered pattern with characteristic spacing of -320 nm.

A second adhesion layer (l nm - Cr) was deposited followed by the first metallic layer 102 (20nm - Au). Tape stripping was used to remove the colloids as described in J. Prikulis, P. Hanarp, L. Olofsson, D. Sutherland, M. Kall, Nano Lett. 2004, 4, 1003. All deposition was performed via automated electron beam evaporation (Lesker PVD225). Deposition could also be performed with reactive sputtering, atomic layer deposition or chemical vapour deposition as is known in the art.

Pixels

Microscale pixels were patterned after Ag deposition by a laser writer (Heidelberg

Instruments DWL 2000). The photoresist (Microposit SI 813) was spin coated at 4000 rpm and baked on a hotplate at 120 °C for 2 min. Pixels for one primary color were then patterned by the 60 mW laser beam after which the sample was developed in developer MF-318 for 50 s. AI2O3 was then deposited and the process was repeated for each primary color. The gold nanohole array was then fabricated as for the other samples over the whole area in one step.

Spectroelectrochemistry

The dark spectrum of the spectrometer (B&WTek CypherX) was recorded with the illumination off and subtracted from subsequent acquisitions. The light source was a 100 W halogen lamp (Newport). The reference intensity was measured as the reflection from a commercial Ag mirror (Thorlabs BBE02-E02). For measurements in the electrochemical cell the additional reflections (e.g., the opposite window) only caused a change in reflectivity of a few percent and were ignored. Reflectivity from metasurfaces was measured with a 10* air objective, while for printed ink a 40 x air objective was used to capture more of the diffuse reflection from the rough paper surface. High angle specular reflection was measured by a spectroscopic ellipsometer (Woilam M2000), which was also used to determine the complex refractive index of the dry polypyrrole. Extinction spectroscopy on ordinary nanohole arrays was performed as described previously. Note that reflectivity is the ratio of measured intensity and reference intensity, while extinction is the logarithm of the inverse of this ratio. A home- built liquid spectroelectrochemistry cell with chloridized Ag as reference electrode and Pt as counter electrode was connected to the potentiostat (Gamry Interface 1000). The open circuit potential between our Ag electrode and a commercial Ag/AgCl standard (World Precision Instruments Dri-Ref) was measured before the experiments (typical difference +80 mV) and the applied potential was adjusted accordingly. Electropolymerizcition

Eleetropolymerization was performed in 0.1 M NaDBS (TCI Chemical, Japan) and 0.1 m pyrrole (Sigma). For optical switching, 0.1 M Li CI (Sigma) was also added, mainly to improve reference electrode stability (although Li+ ions also enter the polypyrrole layer just like Na+). The microscope images were taken with a Thorlabs DCC1645C (CMOS) camera using standard settings. No parts of the photos had been manipulated. In all images that compare "color on" and "color off states, image settings such as brightness and contrast are identical.

Printing reference standards

The three primary colors and "black" were drawn in Paint on Windows 7. The red (R:

237, G: 28, B: 36) color was Hue 238, Sat 205, Lum 125, green (R: 34, G: 177, B: 76) was Flue 92, Sat 163, Lum 99, and blue (R: 0, G: 128, B: 255) was Hue 140, Sat 240, Lum 120. The printer was from HP (Laser Jet Pro 400 color MFP M475dn) and also the ink cartridge (Pink: CE413, Yellow: CE412, Cyan: CE41 1, Black: CE410). Ordinary white A4 paper was used (Future Multitech). The printing was done with "standard" quality settings in Windows.

Experimentation

Reflection Spectra

A device was prepared according to the instructions in the section Fabrication - Layer Structure above. The reflection spectra in air of separate plasmonic structures designed to reflect red, blue and green were measured as described in the section Experimentation - Spectroelectrochemistry. The reflection spectra were measured through a range of

wavelengths (300nm to 800nm) for each sample. The specular reflection were measured at 5 different viewing angles. Normalized reflection spectra results are shown in FIG. 4 for all three samples at viewing angles of 15° and 45°. Normalised reflection spectra for samples at 45°, 55°, 65° and 75° are shown for each of the plasmonic structure samples, red, green blue and green FIG.s 8a-c respectively, and green with no nanostructures in the first metallic layer 102 in FIG. 8d.

As can be seen in FIGs. 4, 7 and 8 each of the plasmonic structures display high reflectivity at narrow viewing angles (0-15°) and maintain high reflectivity at larger viewing angles (45°). The resonant reflection generally shifts to the blue with increasing incident angle. That is, the blue sample becomes a bit more violet (FIG. 8a) and the green samples becomes more cyan (FIG. 8b). As can be seen in FIG. 8c the red sample only becomes slightly- more orange and retains most of its colour reflectivity. The reflection spectra of a green plasmonic structure with no nanostructures in the first metallic layer 102 is shown in FIG. 8d. The peak in reflectivity appears in both cases, showing that the Fabry-Perot interference strongly contributes to the colour. The maximum also shifts similarly to the plasmonic structure comprising nanostructures in the first metallic layer 102. However, there is also a clear increase in the reflectivity of red light for the plasmonic structure not comprising nanostructures in the first metallic layer 102. This is contrary to the green plasmonic structure comprising nanoholes (FIG. 8b) which maintains a low reflectivity for red and results in a stronger primary colour.

As can be seen in FIG.s 4, 7 and 8 the colour of plasmonic structures described herein appear similar except at very high viewing angles (>60°) where scattering is strong.

Determining Polypyrrole thickness

Following the mechanism of PPy synthesis by poly condensation of radical cations, during polymerization each pyrrole monomer donates two electrons for the bond formation as well as one electron for every 4th ring for the oxidation of the polymer and subsequent insertion of the DBS cation. In the polymer each monomer has a weight of 65 gmol^-1 (two hydrogen subtracted upon bond formation) and DBS has a weight of 325 g/mol. Thus the mass deposited on the surface per electron transferred is [8x65 + 325] / 9 /^' NA = 1.56x 10^-22 g. An example of the current during deposition in shown in FIG. 9. The initial current burst is from capacitive charging. After ~20 s the current has stabilized, consistent with a polymer film that grows linearly in time. From these particular data an integrated current of 0.00287 C

(1 .79x 10¹⁶ e-) on 16x7 mm² after 30 s was measured. The background current (no pyrrole present) was negligible (FIG. 9). This means 2.79 μg was deposited and the mass coverage was then 2500 ngcm^-2. The density of the dry PPy film is 1.5 gem^-3, which means that the dry thickness was 17 nm. This was in agreement with profiler sweeps (18 ± 3 nm, not shown). As expected, the deposition rate was found to be dependent on electrode configuration and exact applied voltage. However, the measured current can always be integrated to provide the accurate thickness of the layer as described here. Still, it should be kept in mind that this represents an average value and that care should be taken to achieve uniformity when coating.

A method of non-invasive probes in angular surface plasmon resonance (SPR) was used to determine the thickness of the polymer in the electrolyte. When a steady negative potential (electrochemical SPR) was applied thickness could be estimated because the film was no longer absorbing. As an example, for the device used in FIG 9 the PPy(DBS) thickness in the reduced state was found to be 49 nm, with n = 1.48 and k ~ 4x 10^-4 from SPR Fresnel fits. This represents a volume change of -270% compared to the 17 nm thick dry, as fabricated film, in agreement with previous studies. The thickness in the oxidized state is considerably lower (on the order of 35%) than for the reduced state and thus it should lie somewhere between 25 and 35 nm for the example analysed here. This suggest that the volume fraction of polymer in the film is reasonably close to 50% in the oxidized state (open circuit).

Characterisation of poly pyrrole on ordinary nanohole arrays

To further characterize the polymer-functionalization step and the optical response of the polymer, plasmonic sensing synchronized with electrochemistry was used with nanohole arrays identical to those on the metasurfaces described above ( 150 nm diameter and 20 nm Au film) but prepared directly on glass instead of the AI2O3 spacer layer as shown schematically in FIG. 10b. This was in order to perform the measurements in transmission mode for higher resolution and to acquire information using a nanostructure for which the optical response is easy to interpret. The ordinary nanohole arrays are well studied and the plasmon peak lies outside of the visible for the hole separation and film thickness used here. Thus the plasmonic activity has little influence on the results at the wavelengths of interest and we expect the extinction changes to be associated mainly with the polypyrrole film. This film should, in turn, be identical to that grown on the reflective metasurfaces since the nanohole arrays were prepared in exactly the same way and appeared identical on glass and AI2O3 (verified by electron microscopy). Here it should be noted that the logarithmic units of extinction make different contributions additive and the extinction of an absorbing film is proportional to its thickness (Lambert-Beer law). In summary, using the ordinary nanohole arrays we could monitor changes in extinction and (in the visible) attribute them to the PPy film. (The metasurfaces have essentially zero transmission because of the relatively thick Ag film and could not be analysed in this manner.) FIG. 10a shows the extinction change upon

electropolymerization for three wavelengths. The extinction increases linearly with

polymerization time after -20 s, which is the time to establish linear growth, in reasonable agreement with the current in FIG. 9. Only for the wavelength of 660 nm a slightly longer deviation before the linear increase is seen, which could represent a small contribution from the plasmonic activity in the near infrared. Note that the contrast is highest for blue, which is not the case when switching the colours on/off. This is because here the comparison is against the structure without any PPy, not against the reduced PPy at negative voltage.

FIG. 10c shows the full extinction spectrum changes after electropolymerization and upon applying a negative potential to the polymer-functional ized structure. All spectra are measured in the electrolyte. The peak at -720 nm represents coupling to bonding mode surface plasm ons by the short-range order (quasi -periodicity) of the nanoholes. As expected, the extinction changes for wavelengths around the peak (700-800 nm) showed a more complicated behaviour during electropolymerization because of the contribution from deactivating the plasmonic activity (not shown). As shown in FIG. 10a, after growing the polymer for 5 min, the extinction increases overall due to the absorbing layer and the plasmon peak becomes highly damped. Similar effects contribute to the color changes of the metasurfaces, i.e. light is not just generally absorbed but plasmons are also being switched on and off. Further, it can be noted that there are only small spectral changes upon applying -ί-0.3 V compared with open circuit, as expected. Based on the sensitivity of the plasmon peak to refractive index changes, the resonance shift of -30 nm (FIG. 10c) for the polymer-functionalized nanoholes at negative voltage compared to in the pure electrolyte is consistent with a refractive index change of 0.13, based on the bulk refractive index sensitivity of 230 nm peak shift per refractive index for these ordinary nanohole arrays. Considering that the electrolyte has a refractive index of -1.33, this is in reasonable agreement with the film refractive index of 1.48 determined at a wavelength of 780 nm by SPR (see above). The intrinsic peak shift from the negative potential is much lower (1-2 nm) compared to the shift from the polymer layer.

Using ordinary nanohole arrays how the polymer absorption changed with voltage for different wavelengths could be monitored in detail. An example is shown in FIG. 10 for 490 nm when slowly sweeping the voltage. Two things are noteworthy: The relation between voltage and absorption is not simply linear and there is a saturation in the transparency, i.e. at a certain negative voltage there is no further reduction in absorption (here around -0.8 V). However, the behaviour was different at different wavelengths (data not shown). At 660 nm the absorbance was minimal at -0.9 V. At 520 nm the absorbance was still decreasing when the voltage was further reduced below -1.0 V, suggesting that the contrast for green could be even higher. However, applying reducing potentials lower than -1.0 V (vs Ag/AgCl) damaged the metasurface irreversibly, most likely because of generation of OH- by water hydrolysis, which slowly etches the AI2O3. The stability at strongly negative voltages could be improved by using a pH buffering species or by replacing the A1₂0₃ with a more chemically stable dielectric such as T1O2.

Extinction spectroscopy of polymer-functionalized ordinary nanohole arrays was also used to determine the response time of the optical switching. When measuring reflectivity in real-time, the temporal resolution was 500 ms (FIG. 6) and it could only be concluded that the response time was less than this value, in the extinction measurements a higher intensity reaches the photodetector because of the strong incident beam and the semi-transparent nature of the thin gold film. Therefore, extinction changes could be monitored with a temporal resolution of ~10 ms (FIG, 1 lb) synchronized with the electrochemical measurement. Thus the response could be determined time in more detail for different values of the PPy thickness, as summarized in FIG. 12. The response times clearly lie in the millisecond range as expected considering that the distance that the cations have to travel is less than a micrometer. Further, the response times increase approximately linearly with polymer thickness and are similar overall. Slightly less clear dependence was observed for the response times at the wavelength of 660 nm which can be attributed to a small influence from the plasmonic activity (extinction peak in FIG. 10c) at this wavelength. The response time was not merely a function of PPy thickness but also the electrode configuration, suggesting that further improvement in response time is possible even with this polymer. (In the current electrochemical cell the distance between electrodes is several mm.)

Cyclic voltammetry and transient current

Typical cyclic voltammetry data (10 mVs- 1) is shown in FIG. 13. After 10000 cycles the current is only slightly reduced illustrating the good stability of the polymer and the nanostructures even under the not yet optimized conditions in the experiments. To estimate the power consumption more in detail we looked at the current trace upon switching the voltage between -1.0 V and +0.3 V. The power density is naturally voltage multiplied by current and normalized to electrode area. However, a device 1 comprising a plurality of pixels used for displaying an image would not show a static image and the current would mainly be associated with reversing the polymer oxidation state. FIGs. I4a,b shows the current when switching the voltage on a green sample (230 nm PPy) every 10 s, which we consider a relatively high update frequency for an electronic reading device. The power consumption was calculated by integrating the current trace to get the average current during one cycle. This value is then compared with the background, i.e. the value that the current converges to during a pulse, which represents the leakage (currents not associated with polymer oxidation state) for keeping the color in a steady "on" state. The average current during a 10 s pulse at -1.0 V was found to be ~3 times higher than the leakage (FIGs. I4a,b). The energy density for one switch was -80 Jm^-2. This would mean that the power consumption goes up by a factor of three due to occasional switching (every 10 s). However, this effect will clearly be smaller with lower PPy thickness (less polymer to reduce) and FIGs. 14a,b show the green color plasmonic structure and conductive polymer layer, which has the thickest layer and highest voltage. The factor of three is thus an overestimation with respect to this effect, it should he noted that in an ideal device, almost the entire power consumption should be from switching and the polymer should maintain its oxidation state at open circuit. In our proof-of-concept setup slow reversing of the oxidation state was observed after a negative potential was switched off (FIG. 1 5). The PPy thus exhibits coloration memory as expected, i.e. upon releasing the negative potential it remains reduced, but for a limited time. (This effect can be seen also when switching off in FIG. l i b) FIG. 15 shows the estimated time for the extinction at different wavelengths to increase with 10% (of the total contrast) when switching off (not switching to +0.3 V). Therefore, it is clear that a negative voltage is needed to keep the color "on" and the power consumption is not zero even if a static image would be shown (no switching). It is also clear from FIG. 15 that one could occasionally apply a quick negative voltage pulse to keep the polymer reduced, instead of maintaining a steady negative voltage. This can further reduce power consumption in addition to improving the chemical aspects of the system to prolong coloration memory. Further, one has to take into account that when displaying an image only a fraction of the coloured pixels would be used and not always at their full intensity. For instance, if a device displays some text on a dark background the power usage is obviously orders of magnitude lower compared to a fully white screen. Unless the device should show mainly white images, this effect must result in a reduced typical power density by a factor of at least three when comparing with the "ail pixels on" state. Based on these arguments it can be concluded that the power density values (FIG. 5) are overestimates of the power consumption, at least for a typical electronic reading device usage scenario.

Characterisation of surface plasmon modes

To further characterize the role of surface plasmons calculations were performed on the dispersion relation for the surface plasmon modes in the thin film multilayers. An example is shown in FIG. 16, where a thickness of 80 nm AI2O3, i.e. a blue sample, is used in the calculati on (see schematic in FIG 16a). The permittivity of each material was the same as in the numerical simulations. Multiple metal films give several possible surface plasmon modes, but the nanohoies only enable coupling to the mode with symmetric charge distribution. The dispersion relation for the mode which can be excited is shown in FIG. 16 as mode wavelength (period of plasmon) vs vacuum wavelength (to be compared with incident light). As has been shown in several studies, where we here just cite a few, at normal incidence the surface plasmon is excited when the periodicity of the nanohole array matches the surface plasmon wavelength because this gives momentum matching. Importantly, this holds whether the nanoholes are long-range or only short-range ordered. In these experiments the characteristic spacing, which is the effective periodicity, equals approximately 320 nm (defined by the batch of colloids used in the lithography). Thus one can read from the dispersion relation that the surface plasmon mode will be excited at 540 nm (FIG. 16).

However, due to the presence of holes in the gold film there is always a small error which leads to an underestimation of the resonance wavelength with a few tens of nm. Therefore, one can expect light of wavelength of -570 nm to couple to surface plasmons. FIG 16 also shows the magnetic field distribution (surface plasmons are transverse magnetic) for the plasmon at 570 nm. The field is antisymmetric over the Au film, which means the charge arrangement is symmetric, as expected. Further, there is a high field also at the underlying Ag-AbCh interface, showing that the wave travels through both metals. It should be noted that these calculations include retardation for accuracy. The plasmon at 570 nm had a propagation length of almost 20 μηι for this mode, which means that it has low damping and is excitable. The dispersion relation shows that the surface plasmons for the thin film multilayer system (in contrast to the "ordinary" nanohole arrays above) are excited in the visible region and therefore they can make an important contribution to the colours. We also noticed that the dispersion relation was sensitive to the thickness of AI2O3 as expected from the coupling to the Ag film. Importantly, the thickness of the AI2O3 thus tunes the colour by influencing both the etalon and the surface plasmon. Qualitatively, it is indeed expected that this coupling shifts the plasmon resonance towards higher energy and less loss because Ag has a higher plasma frequency and lower imaginary permittivity, although the absorption in Au at visible wavelengths also contributes to the colours.

Chromacity calculation

The colour range of the RGB metasurfaces were calculated according to the CIE

"standard observer" functions from 1931 , based on human data, denoted χ(λ), ν(λ) and ζ(λ). The chromaticity coordinates are acquired by first calculating the tri stimulus values:

Here R is the reflectivity. The integral range 380 to 780 nm represents the eye-sensitive region in nm. Note that three tristimulus values are calculated for each reflectivity spectrum (RGB from mam text). To get the coordinates in the CXE 1931 diagram (not shown) the tristimulus values are normalized. The coordinates in the CXE 1931 diagram (not shown) were R: (0.4962, 0.4015) G: (0.2116, 0.5572) B: (0.1553, 0.2779). It should be kept in mind that this result is just for the three structures we analyzed in detail in this work. Many more colours are possible simply by tuning the AI2O3 thickness. Further, with other materials, nanohole arrays and conjugated polymers new combinations of pixels and other colour ranges are possible.

Reflectivity of secondary colours

To characterize the secondary colours generated by mixed RGB pixels, FIG. 17 shows the reflectivity measured from the "yellow", "cyan" and "purple" regions of the samples. The spectra were acquired by measuring over a sufficiently large area such that the average of all pixels in each combination were acquired.

Comments of results from experimental section

As is confirmed by the above experimental section, the devices 1 comprising a conductive polymer layer 100, a plasnionic structure 101 provided on a flexible substrate 200 are especially suitable for providing a flexible dynamic multi-colour display. The device 1 has low power consumption and is capable of displaying highly reflective colour pixels, wherein the visibility of each of the pixels is reliably switchable due to the conductive polymer.

Claims

CLAIMS What is claimed is:

1. A reflective display device (1) comprising a conductive polymer layer (100) in communication with an adjacent plasmonic structure (101) adapted to reflect a selected colour from incident light (L), said plasmonic structure (101 ) comprising: a first metallic layer (102) provided on top of a spacer layer (103), and a visible light reflective layer (104) provided below the spacer layer (104), wherein the plasmonic structure (101) is provided on a substrate (200).

2. The reflective display device according to claim 1, wherein incident light (L) to the device (1) transmitted by the conductive polymer layer (100) is substantially reflected at selected wavelengths and scattered or absorbed at non-selected wavelengths by the plasmonic structure (101 ) and wherein incident light (L) is not transmitted through the device (1) as a whole.

3. The reflective display device according to any of claims 1 to 2, wherein the transmission of incident light (L) through the conductive polymer layer (100) is modulated through the application of a potential across the conductive polymer layer (100) therein switching the redox state of the conductive polymer layer (100).

4. The reflective display device (! ) according to any of claims 1 to 3, wherein the peak wavelength of light reflected at selected wavelengths, is shifted by less than about 50 nm for substantially blue and green wavelengths on switching the redox state of the conductive polymer layer (100).

5. The reflective display device (! ) according to any of claims 1 to 4, wherein the device comprises a plurality of plasmonic structures (101) each adapted to reflect a selected colour and wherein each of said plasmonic structures (101) has a spacer layer (103) having a thickness adapted to the selected colour to be reflected.

6. The reflective display device (1 ) according to claim 5, comprising a plurality of conductive polymer layers (100) wherein each of the conductive polymer layers (100), adjacent to and in communication with each of the plasmonic structures (101 ), has a thickness dependent on the selected colour to be reflected.

7. The reflective display device (1 ) according to any of claims 1 to 6, wherein a conductive medium (300) is provided adjacent to and in electrical connection with the conductive polymer layer ( 100) and a separate reference electrode.

8. The reflective display device (1) according to any of claims 1 to 7, wherein the substrate (200), the plasmonic structure (101) and the conductive layer ( 100) are non-rigid.

9. The reflective display device (1) according to any of claims 1 to 8, wherein the spacer layer (103) has a thickness from 30nm to l O0nm, such as 40 urn - 95 nm, or 43 - 53 nm, 78 - 88 nm, 88 - 98 nm, or about 48 nm, 83 nm, or 93 nm.

10. The device according to any of claims 1 to 9 wherein the first metallic layer (102) is provided with a plurality of nanostructures ( 120) such that incident light (L) is scattered.

11. The device according to claim 10 wherein the first metallic layer is comprised of gold and wherein the nanostructures ( 120) are substantially circular nanoholes.

12. A flexible reflective display device (1) according to any of claims 1 to 1 1.

13. A display assembly comprising a plurality of reflective display devices ( 1) according to claims 1 to 12, wherein at least the first metallic layer (102) of each of the plasmonic structures ( 101) of the plurality of devices (1) is electrically isolated from the first metallic layer (102) of adjacent plasmonic structure(s) (101) and wherein the redox state of, and therein transmission of light through, the conductive polymer layer (100) above each plasmonic structure (101) is modulated by the application of an electric potential between the first metallic layer (102) of each plasmonic structure (101) and a reference electrode.

14. The display assembly according to claim 13, comprising a plurality of pixels wherein each pixel is a triplet of reflective display devices (1) arranged adjacent to one another, and wherein each of the first, second and third devices has a plasmonic structure (101) arranged for reflecting a unique selected colour.