Abstract
We disclose herein a hetero-structure comprising: a curved material; at least one layer of a first material rolled around the curved material; at least one intermediate layer rolled on the at least one layer of the first material; and at least one layer of a second material rolled around the at least one intermediate layer.
Claims
1. A hetero-structure comprising: a curved material; at least one layer of a first material rolled around the curved material; at least one intermediate layer rolled on the at least one layer of the first material; and at least one layer of a second material rolled around the at least one intermediate layer.
2. A hetero-structure according to claim 1, wherein the curved material comprises a fibre comprising glass, metal, carbon, nylon, polyester, cotton wools or a mixture thereof.
3. A hetero-structure according to claim 1, wherein the first and second materials each comprise an electrically conducting material.
4. A hetero-structure according to claim 1, wherein at least one of the first and second materials comprises graphene, boron nitride, transition metal dichalcogenides, dopant structures, a binary alloy or a ternary alloy, and/or wherein at least one of the first and second materials comprises chemical vapour deposition (CVD) or physical vapour deposition (PVD) single layer graphene (SLG).
5. (canceled)
6. A hetero-structure according to claim 1, wherein the first material comprises a plurality of rolled layers, and wherein the second material comprises a plurality of rolled layers, and/or wherein the first material comprises at least six layers rolled on top of one another, and wherein the second material comprises any of one to six layers rolled on top of one another.
7. (canceled)
8. A hetero-structure according to claim 1, wherein the at least one intermediate layer is an insulation layer, and/or wherein the at least one intermediate layer comprises a mixture of aluminium oxide (Al.sub.2O.sub.3) and a conformal coating of pin-hole-free Parylene C.
9. (canceled)
10. A hetero-structure according to claim 8, wherein the at least one layer of the first material is configured to operate as a gate region and the at least one layer of the second material is configured to operate as a channel region, and/or wherein the hetero-structure further comprises a source region and a drain region located on the at least one layer of the second material, the channel region being located between the source and drain regions.
11. (canceled)
12. A hetero-structure according to claim 8, further comprising a photoactive material located on the channel region, and/or wherein the photoactive material comprises a doped perovskite solution.
13. (canceled)
14. A hetero-structure according to claim 8, wherein the hetero-structure is a gas sensing device, and/or wherein the gas sensing device is configured to detect a gas by detecting a change in resistance of the channel region.
15. (canceled)
16. A hetero-structure according to claim 8, wherein the hetero-structure is a phototransistor.
17. A hetero-structure according to claim 1, wherein the at least one intermediate layer comprises a plurality of semiconducting layers, and/or wherein the plurality of semiconducting layers comprise a hole transport layer and a photoactive layer over the hole transport layer.
18. (canceled)
19. A hetero-structure according to claim 17, further comprising a first electrode disposed on the at least one layer of the first material and a second electrode disposed on the at least one layer of the second material.
20. A hetero-structure according to claim 17, wherein the hetero-structure is a light emitting diode (LED) or a solar cell.
21. (canceled)
22. A hetero-structure according to claim 1, wherein the hetero-structure is an imaging sensor and/or an ultraviolet (UV) detector and/or a strain sensor, and/or an actuator, and/or an optic coupler.
23. A rolled device comprising the hetero-structure according to claim 1.
24. (canceled)
25. A method of depositing one or more layers of a first material on a fiber comprising: (i) providing a sheet of the first material in a liquid; (ii) fishing the sheet of the first material out of the liquid and onto a transfer support; (iii) transferring the sheet of the first material from the transfer support onto the fiber by rolling the fiber; and (iv) optionally cutting the sheet of the first material.
26. A method as claimed in claim 25, wherein said fiber comprises a polymer, glass, metal or a mixture thereof.
27. A method as claimed in claim 25, wherein said sheet of first material comprises an electrically conducting material, and/or wherein said sheet of first material comprises graphene.
28. (canceled)
29. A method as claimed in claim 25, wherein said sheet of first material comprises a protective film.
30. A method as claimed in claim 25, further comprising the step of providing a sheet of the first material on a support and removing the material from the support.
31. (canceled)
32. A method as claimed in claim 25, wherein said liquid has a surface tension of 10-100 mN/m, and/or wherein said transfer support and said liquid has a contact angle of 20-50°.
33. (canceled)
34. A method as claimed in claim 25, wherein said transfer support comprises polyethylene terephthalate (PET), polyethylene naphthalate (PEN) or glass, and/or wherein a plurality of layers of first material is deposited on the fiber by rolling the fiber.
35. (canceled)
36. A method as claimed in claim 25, wherein a layer of a second material is deposited on top of the first material, and/or wherein a further layer of said first material is deposited on top of said second material by repeating steps (i)-(iv).
37. (canceled)
38. (canceled)
39. (canceled)
40. (canceled)
41. (canceled)
42. An article or a device comprising a fiber obtainable by the process of claim 25.
43. (canceled)
Description
[0152] The invention will now be described in detail with reference to the following Figures and Examples wherein:
[0153] FIG. 1 is a schematic drawing of the method of the invention;
[0154] FIG. 2 shows (a) an optical microscopy image, (b,c) SEM images of transferred CVD SLG around a glass fiber, (d) conductivity versus number of rolled CVD SLG and (e,f) Raman characteristics of CVD SLG before and after transfer;
[0155] FIG. 3 illustrates a step-wise fabrication process of fiber-based hybrid rolled graphene-perovskite phototransistors;
[0156] FIG. 4 illustrates a schematic representation of a phototransistor and/or a sensing device;
[0157] FIG. 5 illustrates a schematic representation of a light emitting diode (LED) and/or solar cell;
[0158] FIG. 6 illustrates in (a) atomic force microscopy of perovskite films formed on top of the rolled SLG channel shown in the optical microscopy image, (b) scanning electron microscopy image of perovskite film formed on the rolled graphene based phototransistor, (c) absorption spectra of pristine perovskite and hybrid graphene-perovskite films, (d) photoluminescence spectra of pristine perovskite and hybrid graphene-perovskite films upon excitation at 514 nm, (e) time resolved PL spectra of pristine perovskite and hybrid graphene-perovskite films at excitation wavelength of 400 nm;
[0159] FIG. 7 illustrates in (a) the source-drain current as a function of applied bias at dark condition before and after deposition of perovskite on channel layer, (b) the transconductance characteristics (Vsd=0.5V) of fiber based photodetector at dark condition, (c) Illumination power dependent transfer characteristics of the fiber based phototransistor at fixed wavelength of 488 nm, (c) corresponding drain photocurrent as a function of applied gate voltage, (d) photocurrent at Vg=+2V of the fiber based phototransistor as a function of applied bias at different illumination power, (e) corresponding external responsivity as a function of optical power;
[0160] FIG. 8 illustrates in (a) source-drain current as a function of applied bias at dark condition before and after deposition of perovskite on channel layer, (b) the transconductance characteristics (Vsd=0.5V) of fiber based photodetector at dark condition, (c) Corresponding drain photocurrent as a function of applied gate voltage, (d) photocurrent at Vg=0V of the fiber based phototransistor as a function of applied bias at different illumination power, (e) corresponding external responsivity as a function of optical power, (f) noise measurements for the fiber phototransistors with different rolling numbers (1 and 6 layers) of rolled SLG;
[0161] FIG. 9 illustrates in (a) temporal photocurrent response of the fiber based hybrid rolled graphene-perovskite photodetector under alternating dark and light illumination, (b) wavelength dependent photoresponse characteristics of fiber based hybrid graphene-perovskite phototransistor. Inset, shows corresponding detectivity of the device at each wavelength.
[0162] FIG. 10 illustrates (a) bending radius versus the change in the photocurrent normalized according to the photocurrent at the flat state. (b) device performance under different bending cycles. Inset shows a schematic illustration of a three-point bending setup of fiber based phototransistor. L indicates the chord of circumference connecting two ends and H is the height at the chord midpoint. (c) performance of the device at different number of washing cycles. Laundry washing were followed according to AATCC standard test procedures. After each cycle, device was dried and photocurrent values were measured. (d) Raman spectra of encapsulated perovskite at different washing cycles.
[0163] FIG. 11 illustrates a set of gas sensing experimental results for device having channel based on 1 layers rolled CVD SLG;
[0164] FIG. 12 illustrates a further set of gas sensing experimental results for device having channel based on 6 layers rolled CVD SLG; and
[0165] FIG. 13 illustrates the dependency of number of layers on gas sensitivity and selectivity.
DETAILED DESCRIPTION OF THE FIGURES
[0166] The preferred method of the invention is illustrated in FIG. 1. Referring to FIG. 1a, this shows a sheet of a first material, specifically a CVD single layer graphene sheet, on a copper support. In FIG. 1a, the sheet is shown without a protective film on top of the CVD single layer graphene sheet, but as described above, a protective film is preferably present. FIG. 1b shows a fast electrochemical delamination. The CVD single layer graphene sheet is made the cathode and a platinum sheet (not shown) the anode. The sheets are both immersed in an aqueous solution of potassium hydroxide (e.g. 0.1 M concentration) and a voltage (e.g. 1-2 V) is applied. Hydrogen bubbles generate at the surface and aid delamination from the copper support which typically occurs within 10-60 s. After washing the delaminated CVD single layer graphene sheet is placed in a liquid as shown in FIG. 1c. From the liquid, the CVD single layer graphene sheet is fished onto a transfer support. Typically this means that the transfer support is dipped into the liquid, underneath the CVD single layer graphene sheet so that the support is brought into contact with the graphene exposed by the electrochemical delamination process. Prior to bringing the CVD single layer graphene sheet into contact with the transfer support, preferably the fiber is laid across the top edge of the transfer support. Thus when the CVD single layer graphene sheet contacts the transfer support, it also contacts the fiber. Once the CVD single layer graphene sheet and transfer support, and preferably the fiber, are in contact, the structure is preferably lifted out of the liquid. Subsequent rolling of the fiber then causes the CVD single layer graphene sheet to slide along the transfer support and to roll around the fiber. The sliding motion preferably causes a slight stretching of the CVD single layer graphene sheet which means that it is wrinkle and crease free. The rolling of the fiber causes layers of CVD single layer graphene sheet to form on the fiber is a controlled manner. Advantageously the layers confirm to the shaper of the fiber. Additionally, due to the stretching that occurs, there are few, if any, wrinkles or creases, in the layers formed. Finally, FIG. 1d presents the scale up approach for continuous coating of the passing fibers with the floating layers on the move.
[0167] FIG. 3 illustrates an example of a step-wise fabrication process of fiber-based hybrid rolled graphene-perovskite phototransistors or hetero-structures (inset image shows the rolled SLGs sandwiched in between the dielectric and perovskite layers). Although graphene is used in this example, other suitable materials as discussed in this specification can also be used instead of graphene. As can be seen in FIG. 3, a curved core fiber or a suitable curved material 400 is provided and then a single layer graphene 405 (or the first material) is rolled or wrapped around the core fiber 400. Then, in one example, a dielectric or insulation material 410 is formed on the wrapped graphene 405. After this, in one example, a further graphene layer 415 (or the second material) is wrapped around the dielectric material 410. A channel region is normally formed in second material or second graphene rolled layer 415. Finally, in one example, a photoactive material 420 is formed on the second rolled material (or graphene in one example) 415. In this example, the hetero-structure forms a phototransistor, but it will be appreciated that the same structure can be used as a sensing device, such as a gas sensing device.
[0168] FIG. 4 illustrates a schematic representation of a phototransistor and/or a sensing device. Many features of the device of FIG. 4 are the same as those shown in FIG. 3 and therefore carry the same reference numbers. However, in the device of FIG. 4, gate 505, drain 510 and source 515 contacts are provided. Drain 510 and source 515 are formed on the second rolled material (e.g. graphene) 415 and the photoactive material 420 is located between the drain 510 and source 515. The gate contact 505 is formed on the first rolled material (e.g. first graphene rolled layer) 405.
[0169] FIG. 5 illustrates a schematic representation of a light emitting diode (LED) and/or solar cell. The device includes a core fiber 600 and a first material (e.g. a first graphene layer) 605 rolled around the core fiber 600. A hole transport layer 610 is formed on the first rolled material 605 and a photoactive material 615 is formed on the hole transport layer 610. A second rolled material (e.g. a second graphene layer) 620 is wrapped around the photoactive material 615. A first (bottom) electrode 625 is formed on the first rolled material 605 and a second (top) electrode 630 is formed on the second rolled material 620. This hetero-structure can be used as a LED and/or a solar cell device.
Materials:
[0170] PMMA-coated single layer graphene (SLG) on a copper support was obtained from Sigma-Aldrich or Fisher Scientific;
Ammonium persulfate (APS) was obtained from Sigma-Aldrich or Fisher Scientific;
Glass fiber was obtained from Sigma-Aldrich or Fisher Scientific;
Transfer supports made of each of PET, PEN, quartz and glass were obtained from Sigma-Aldrich or Fisher Scientific.
Characterisation Methods
[0171] Scanning electron microscopy (SEM) was carried out using High Resolution SEM Magellan;
Optical microscopy was carried out using Thorlabs equipped microscope;
I-V measurements for device characteristics was carried out using Keithley 2612B.
Temporal photocurrent response measurements were carried out using Keysight oscilloscope.
The AFM measurement were performed in a Bruker Dimension Icon with a scanning rate of 0.972 Hz in a tapping mode.
The Raman measurement were performed via a Horiba Jobin Yvon HR800 spectrometer equipped with a 50× objective. The laser power is kept below 100 μW.
Methods
Delamination of PMMA-coated SLG
(i) Chemical Etching
[0172] 3×6 cm CVG grown graphene on copper (both sides) was sliced into 1×1 cm pieces, PMMA was then spin coated onto the top side at 4000 rpm for 40 s to produce protective coatings, ˜400 nm in thickness. Afterwards, oxygen plasma was used to remove any excess graphene from the backside of the copper. The pieces were then submerged in 150 ml of aqueous ammonium persulfate solution ((NH.sub.4).sub.2S.sub.2O.sub.8, 0.5 mol/L) for 2 hours at 25° C. for complete copper dissolution. The resulting free-standing PMMA-coated graphene layers were washed according to the procedure described below.
(ii) Electrochemical Delamination
[0173] A two-electrode system was used, with the PMMA-coated graphene sheet as the cathode (negatively charged) and a platinum sheet as the anode (positively charged). The system was submerged in a 0.1 M aqueous solution of KOH, and a voltage of 1-2 V was applied. The average current was 0.1-1 mA/cm.sup.2. Hydrogen bubbles formed at the copper-graphene interface and facilitated the mechanical delamination the graphene from the copper support. After complete delamination, which typically took 10-60 s, the free-standing PMMA-coated graphene was then washed according to the procedure described below.
[0174] After delamination by either chemical etching or electrochemical delamination, the free-standing PMMA-coated graphene sheets was then cleaned by dipping in fresh deionised water for 20 minutes, either 2 or 3 times.
Fishing and Rolling onto a Fiber, Including Removal of the Protective Film
[0175] The rolling process consisted of two main steps. First, the edge of the PMMA-coated graphene sheet was “grabbed” by the top of the glass fiber (125 micron diameter with 1 cm target length coverage). Specifically, the fiber was placed on top of the transfer support and a top edge of the floating graphene sheet was fished onto the fiber. The remainder of the graphene sheet lies on the surface of the transfer support. Second, and after waiting for a few seconds for the graphene sheet to dry, the fiber was rotated at speeds ranging from 1-10 rpm. The rotation of the fiber slides the graphene sheet along the transfer sheet and onto the rotating surface of the fiber. The sheet is stretched in the process and as a result the graphene layers formed on the fiber are wrinkle-free and crease-free and show a high level of conformity with the fiber.
[0176] A different number of layers was rolled depending on the end application of the fiber. After reaching the desired level of rolling, the sheet was cut by pulling it away from the transfer support.
[0177] Finally, the PMMA-coating on the graphene was removed by immersing the coated fiber in acetone for 15 min followed by rinsing with isopropanol for at least 10 min and drying under N.sub.2 gas flow. Annealing at 300° C. can be carried out to ensure the complete removal of any residues. FIG. 2 shows scanning electron microscopy and optical microscopy images of single layer graphene rolled around a sample fiber.
Device Fabrication Process
[0178] The following description is discussed as examples of implementations of the hetero-structure described above.
[0179] The demonstrated transfer process is utilized to fabricate the main components (gate and channel) of large-area and flexible field effect transistors (FETs). Moreover, the source and drain electrodes are generally made by inkjet printing technology using silver nanoparticle inks. Referring to FIG. 3 again, this presents a step-wise fabrication schematic of the FET devices which are used to construct the rolled CVD SLG-perovskite hybrid phototransistors around fibers. Rolled CVD SLG (e.g. 6 times) was employed as the gate electrode. Then, 150 nm of Al.sub.2O.sub.3 plus 200 nm Parylene C were deposited as the dielectric layer. Al.sub.2O.sub.3 deposition was performed using atomic layer deposition (ALD), which offers high conformity.
[0180] During the preliminary experiments, the sole use of Al.sub.2O.sub.3 as dielectric resulted in a premature failure due to various mechanical stress/strain applied to the sample throughout the device fabrication process. To address this, an additional conformal coating of 200 nm pin-hole-free Parylene C was applied via SCS parylene coater. Use of Parylene as a reliable dielectric choice is reported with promising prospects for flexible electronics. Since Parylene C is applied as gas and evaporated at ambient temperature under vacuum, it ultimately covers all the available surfaces, providing complete and uniform encapsulation. Consequently, this step further improved the dielectric robustness against deformations during manufacturing and significantly reduced the risk of short circuit.
[0181] Afterwards, in one example, another CVD SLG was rolled as the channel material followed by printing silver contacts to establish source and drain of the device (channel length for example: 1000 μm width: 120 μm). Without any lithography step (typically done for channel patterning), in one example, a state of the art CH.sub.3NH.sub.3PbI.sub.3 perovskite was deposited via a simple spin coating procedure on top of the rolled CVD SLG surface. The resulting film yielded in a uniform coverage with an average roughness of 20 nm obtained by atomic force microscopy (AFM) (FIG. 6a). SEM image of perovskite films on rolled CVD SLG is displayed in FIG. 6b. Moreover, FIG. 6c depicts the absorption spectra of perovskite, SLG and hybrid SLG-perovskite systems. The steady state photoluminescence (PL) spectra of the perovskite and hybrid SLG-perovskite configurations (on quartz substrates) are shown in FIG. 6d. Under the same experimental conditions, both perovskite and hybrid SLG-perovskite exhibited PL peak at ˜762 nm arising from the perovskite band gap. However, the PL quantum yield of pristine perovskite is higher than SLG-perovskite hybrid. In other words, the PL intensity (integrated area under the curve) of the SLG-perovskite with respect to perovskite itself was quenched by nearly 96%.
[0182] Such a significant PL quenching could be attributed to effective charge carrier transfer between perovskite and graphene layer. To explore this further, time resolved PL analysis was done at ambient temperature (FIG. 6e) which indicates faster decay time in the case of hybrid SLG-perovskite compared to pristine perovskite; which confirms the transfer of charges from perovskite to graphene.
Photo-Response Characterisation
[0183] In the next step, the photoresponse characteristics of the fabricated phototransistors were investigated. In all devices, at least 6 layers of graphene were rolled as the gate electrode. By controlling the rolling process, the effect of layer dependency in case of graphene as the channel material was investigated. To begin with, 1-2 layers SLG rolling was explored. The linear dependency of current to voltage indicates the low ohmic contact of the electrodes with the channel layer, see FIG. 7a. By depositing perovskite on top of the graphene-based channel, the current values increase suggesting the transfer of holes from perovskite to the channel layer. Similarly, the transconductance curves of as fabricated fiber phototransistors without illumination are displayed in FIG. 7b before and after perovskite deposition. The gate voltages in the range of −2V to 2V were applied here to avoid the dielectric breakdown. In graphene-only devices, the charge neutral point emerged in larger positive gate voltages according to the drain current modulation. This reveals a p-doped channel behaviour; typically caused by water trapping (or absorption) underneath (or above) the rolled SLG layer. The drain current regime for the rolled SLG-perovskite transistor is similar to that of graphene-only versions though with a major shift to higher gate voltages. This could suggest that p-doping effect in pristine graphene is strengthened due to the hole transfer from perovskite to the hole dominated graphene channel. This is consistent with FIG. 7a which presents source-drain current as a function of applied bias. The increase of hole current in the graphene channel further confirms the transfer of holes (electrons) from perovskite (graphene) to graphene (perovskite).
[0184] FIG. 7c demonstrates transconductance measurements (drain current vs gate voltage) under different light intensities at 488 nm. In one example, all measurements were conducted at a fixed drain voltage of 0.5V where the black line indicates the dark current. Due to the ultrafast charge re-combination in graphene, the photo-generated carriers do not contribute to the observed photocurrent. As shown in FIG. 7c, the drain current under illumination shifted towards more positive gate voltages. This is attributed to the additional gating effect generated by the photo-excited electrons in perovskite. The resulting electron-hole pairs combined with the existing holes in perovskites valence band are transferred to graphene (i.e., electron transfer from graphene to perovskite); inducing a hole current in graphene channel through capacitive coupling. This is generally in agreement with the quenched PL intensity measured in hybrid SLG-perovskite (FIG. 6c).
[0185] Moreover, FIG. 7d suggests that higher photocurrents can be achieved by increasing the gate voltage to +2V. Elevating gate voltages induces further p-doping in graphene channel resulting in a strong electric field at the graphene-perovskite junction; which favours hole transfer to SLG. Consequently, larger photocurrents are observed at elevated gating due to increased transfer rate of the photo-generated holes.
[0186] The photocurrent (I.sub.ph=I.sub.light−I.sub.dark) of the fiber-based hybrid devices at V.sub.g=+2V is summarized in FIG. 7e under different illumination intensities. These values were obtained by sweeping drain voltage from −1 to 1V. Consistent with the above-mentioned photo-gating mechanism, the illumination power rise leads to higher photocurrents. A larger number of photo-generated electrons at higher illumination power are trapped in the perovskite inducing more negative voltages within the hybrid structure. The corresponding external responsivity as a function of illumination intensity is displayed in FIG. 7f. External responsivity is defined as R.sub.ext=(|I.sub.sight−I.sub.dark|)/(Popt.APD/A.sub.0)) where I.sub.light and I.sub.dark are the device current under illumination and in the dark, respectively, P.sub.opt is the impinging optical power and APD/A.sub.0 is a scaling factor to take into account the active device area. Subsequently, for example, the fiber-based hybrid phototransistor exhibited external responsivity up to 376 A/W under illumination power of 750 pW and applied bias of 1 V. Photoconductive gain can be estimated as the ratio between τ.sub.life (excess carrier lifetime) to τ.sub.transit (transit time). For higher voltage applied across the source and drain (V.sub.sd), the free carriers drift velocity V.sub.d in graphene channel increases lineally until reaching saturation caused by the carrier scattering with optical phonons. Since t.sub.tr=L/V.sub.d where t.sub.tr is transit time and L is the channel length, the increase in drift velocity (V.sub.d) results in shorter transit time (t.sub.tr). Thus, photoconductive gain is generally expected to grow linearly with V.sub.sd, results in higher external responsivity. Therefore, all measurements were done at V.sub.sd=1 V to keep the device operation in the linear (ohmic) regime thus eliminating the nonlinear dependence of V.sub.d on V.sub.sd.
[0187] Next, for example, the number of rolled graphene for the channel was increased to 6 layers while all other conditions (dielectric and gate electrode were kept similar). The experiments for the as-fabricated devices in the absence of light are presented in FIG. 8a, b. Similar to the previous device, there is increasing in the drain current and gate voltage shifted to the right after deposition of perovskite on graphene-based channel layer. This variation is attributed to the change in graphene's Dirac point; which generally suggests that the p-type doping in graphene is strengthened due to transfer of holes from perovskite to graphene.
[0188] In one example, the power dependent transconductance measurements at 488 nm under drain voltage of 0.5 V are presented in FIG. 8c. According to the results, the drain current increased under light illumination and shifted to higher gate voltages. The photo-generated charges are separated at the graphene-perovskite interface due to the electric field buildup. The holes are transferred to the graphene sheet while electrons are trapped in perovskite creating the photo-gating effect. Consequently, larger photocurrents were generally observed in 6 layer rolled SLG-perovskite phototransistors compared to those of on 1-2 layers SLG configurations.
[0189] FIG. 8d shows the photocurrent at zero gate voltage and sweeping drain voltage between −1 to 1 V under different illumination intensities. The corresponding responsivity shows extremely high value of 22000 A/W at 1V at 488 nm with a power of 750 pW as shown in FIG. 8e. To further characterize the device, the detectivity was recorded. The measured noise in the dark current shows a strong 1/f component in FIG. 8f. Considering the measurement at 1 Hz frequency, this leads into a noise equivalent power of 7.9×10.sup.−9 W/Hz.sup.1/2 with the corresponding detectivity of 10.sup.7 jones (cm.Math.Hz.sup.1/2/W) for phototransistor based on one layer rolled SLG. Similarly, the noise measurement for the phototransistor based on 6 layers rolled SLG were applied, resulted in noise equivalent power of 2.8×10.sup.−10 W/Hz.sup.1/2 with the corresponding detectivity of 10.sup.6 jones (cm.Math.Hz.sup.1/2/W). The noise current in the shot noise limit scales as in =(2 qldB).sup.1/2, where B is the electrical bandwidth, q is the electron charge and Id is the dark current in the device. As a result, the directivity in our device based on the shot noise limit can be expressed as D*=R(AB).sup.1/2/in where A is the active area of the device, B is the electrical bandwidth, and R is the external responsivity. The shot noise limited detectivity were calculated as 10.sup.12 and 10.sup.13 jones for one layer rolled SLG and 6 layers rolled SLG phototransistors, respectively.
[0190] Generally speaking, the obtained responsivities for this hybrid device are seven orders of magnitude higher than pristine graphene (1 mA/W) and four orders of magnitudes higher than pristine perovskite (3 A/W). Such an unprecedented performance is a product of an effective charge transfer and photoconductive gain mechanism. The photoconductive gain can be calculated as follow; G=τ.sub.life/τ.sub.transit=(τ.sub.life/[L.sup.2/μV.sub.sd]); where G stands for photoconductive gain, τ.sub.life is excess carrier lifetime, τ.sub.transit is transit time, L is the channel length, μ is the mobility of the channel and V.sub.sd is the voltage applied between source and drain.
[0191] To calculate the τ.sub.life, the temporal photocurrent response of graphene-perovskite was measured in FIG. 9a. The results reveal that a rise time of 5 ms and a fall time less than 35 ms can be achieved without applying short gate pulses; which make the fabricated fiber device suitable for image sensing applications. Gain values in the range of at V.sub.sd=1V were obtained at different optical powers at 488 nm. FIG. 9b presents the spectral responsivity of the hybrid SLG-perovskite photodetector ranging from visible up to 870 nm (near infrared). This photoresponse spectra closely follows the intrinsic absorption of graphene/perovskite films shown in FIG. 9 highlighting the role of perovskite as a strong light absorbing layer in which photoexcited charged are mainly produced.
Device Stability and Resilience
[0192] Finally, as an example, the robustness and stability of the as-prepared fiber photodetectors were examined via torturous mechanical bending and washability tests. The inset in FIG. 10a shows the variation in the photocurrent at different bending radius. Inset in FIG. 10b depicts the schematic of the three-point bending setup (Deben Microtest) where the photocurrent is measured as a function of bending cycles. In this configuration, the bending radius (R.sub.b) is calculated by the following formula R.sub.b=[H.sup.2+(L/2).sup.2]/2H where H is the height at the chord midpoint and L is the chord of circumference connecting two ends of the arc. The photocurrent was first measured at a flat position (no bending), and the device was then bent up to 9 mm radius curvature for 100 bending cycles (FIG. 10b). The steady ratio of bending to flat as a function of bending cycles suggests a stable performance after 100 bending tests with standard deviation of 0.02.
[0193] Afterwards, the washability of the samples was investigated as one of the main criteria for applications in the electronic textiles. For this, AATCC test protocol was carefully followed as suggested by numerous studies on e-textiles. Here, two encapsulation techniques were explored to protect the device during washing tests (mainly polymer-based). Initially, the fibers were conformally coated with a thin layer of Parylene C (1 μm); known as an excellent moisture barrier with high thermal (melting point 563 K) and UV stability while being biocompatible. The device was then coated further with polydimethylsiloxane (PDMS) for further waterproofing and resistance against harsh tension/compression introduced during washing. For this, the elastomer base and the curing agent were mixed with 1:0.1-1 ratios and spread on top of the fiber with 0.1-1 mm thickness. The samples were then cured at 80° C. for 30 minutes to harden the polymer coating before placing in the washing machine (SKYLINE rotate washing color) at 40° C. for 30 washing cycles, each programmed for 45 min. The samples were drip dried in a ventilated oven afterwards and tested for photo-responsivity. As shown in FIG. 10c, the devices were still functional after 30 washing cycles with slight drop in the photocurrent compared to the initial value (less than 20%) upto 30.sup.th cycles. This could be attributed to the gradual degradation of the electrodes during extensive washing. Also, Raman investigation of encapsulated perovskite at different washing cycles was conducted, shown in FIG. 10d. As it can be seen, no degradation of perovskite to PbI.sub.2 was monitored at any washing cycles, confirming the robustness of our proposed encapsulated layer. Furthermore, Parylene C emerged as the integral part of the device encapsulation/protection owing to its transparency and flexibility towards effective sealing of the devices and their accompanying components.
Gas Sensing Performance FIG. 11 presents a series of graphs demonstrating the high sensitivity of the 1-2 layers rolled SLG towards various gases at room temperature. It also confirms that the same device architectures depicted in FIGS. 3-4 are capable of sensing different gases. The graphs show a significant correlation between ohmic resistance (or its change in %) of the rolled SLG against various gases at different concentration levels including relative humidity (RH %), ammonia (NH.sub.3) and nitrogen dioxide (NO.sub.2).
[0194] FIG. 12 shows similar testing performed with the 6 layers of rolled SLG as the channel. The notable observation is the high selectivity towards NO.sub.2 with a dramatic decrease of sensitivity to RH % and NH.sub.3.
[0195] FIG. 13 compares the above described trends observed in the 1 layer and 6 layer rolled SLGs for relative humidity (RH %) at 60%, carbon dioxide (CO.sub.2) at 600,000 ppm and ammonia (NH.sub.3), sulphur hexafluoride (SF.sub.6) and nitrogen dioxide (NO.sub.2) at 60 ppm. This graph further demonstrates the high sensitivity of the SLG with few layers and the high selectivity of the SLG towards nitrogen dioxide as the number of layers is increased. Inset on this graph presents the relative change in response towards the aforementioned gases as the number of layers increases.
[0196] Although the invention has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.
