Mid and far-infrared nanocrystals based photodetectors with enhanced performances

Abstract

Disclosed is a plurality of metal chalcogenide nanocrystals coated with multiple organic and inorganic ligands; wherein the metal is selected from Hg, Pb, Sn, Cd, Bi, Sb or a mixture thereof; and the chalcogen is selected from S, Se, Te or a mixture thereof; wherein the multiple inorganic ligands includes at least one inorganic ligands are selected from S.sup.2, HS.sup., Se.sup.2, Te.sup.2, OH.sup., BF.sub.4.sup., PF.sub.6.sup., Cl.sup., Br.sup., I.sup., As.sub.2Se.sub.3, Sb.sub.2S.sub.3, Sb.sub.2Te.sub.3, Sb.sub.2Se.sub.3, As.sub.2S.sub.3 or a mixture thereof; and wherein the absorption of the CH bonds of the organic ligands relative to the absorption of metal chalcogenide nanocrystals is lower than 50%, preferably lower than 20%.

Claims

1. A plurality of metal chalcogenide nanocrystals coated with multiple organic and inorganic ligands; wherein said metal is selected from Hg, Pb, Sn, Cd, Bi, Sb or a mixture thereof; and said chalcogen is selected from S, Se, Te or a mixture thereof; wherein said multiple inorganic ligands comprise at least one inorganic ligand selected from S.sup.2, HS.sup., Se.sup.2, Te.sup.2, OH.sup., BF.sub.4.sup., PF.sub.6.sup., Cl.sup., Br.sup., I.sup., As.sub.2Se.sub.3, Sb.sub.2S.sub.3, Sb.sub.2Te.sub.3, Sb.sub.2Se.sub.3, As.sub.2S.sub.3 or a mixture thereof.

2. The plurality of metal chalcogenide nanocrystals according to claim 1, wherein the absorption of the organic ligands relative to the absorption of coated metal chalcogenide nanocrystals is lower than 50%.

3. The plurality of metal chalcogenide nanocrystals according to claim 1, wherein said plurality of metal chalcogenide nanocrystals exhibits an optical absorption feature in a range from 3 m to 50 m and a carrier mobility not less than 1 cm.sup.2V.sup.1s.sup.1.

4. The plurality of metal chalcogenide nanocrystals according to claim 1, wherein said metal is selected from Hg or a mixture of Hg and at least one of Pb, Sn, Cd, Bi, Sb; and said chalcogen is selected from S, Se, Te or a mixture thereof; provided that said metal chalcogenide nanocrystals coated with inorganic ligands is not HgTe coated with As.sub.2S.sub.3.

5. The plurality of metal chalcogenide nanocrystals according to claim 1, wherein the inorganic ligands are As.sub.2Se.sub.3.

6. The plurality of metal chalcogenide nanocrystals according to claim 1, wherein said metal chalcogenide nanocrystals are doped.

7. A method for manufacturing a plurality of metal chalcogenide nanocrystals according to claim 1, said method comprising the following steps: providing a metal carboxylate in a coordinating solvent; admixing within said solution a chalcogenide precursor at a temperature ranging from 60 C. to 130 C.; isolating the metal chalcogenide nanocrystals; and coating the isolated metal chalcogenide nanocrystals with multiple inorganic ligands.

8. The method for manufacturing colloidal metal chalcogenide nanocrystals according to claim 7, wherein the metal carboxylate is a metal oleate or a metal acetate.

9. The method for manufacturing colloidal metal chalcogenide nanocrystals according to claim 7, wherein the coordinating solvent is selected from a primary amine.

10. The method for manufacturing colloidal metal chalcogenide nanocrystals according to claim 7, wherein the primary amine is oleyamine, hexadecylamine or octadecylamine.

11. The method for manufacturing colloidal metal chalcogenide nanocrystals according to claim 7, wherein the chalcogenide precursor is selected from trioctylphosphine chalcogenide, trimethylsilyl chalcogenide or disulfide chalcogenide.

12. The method for manufacturing colloidal metal chalcogenide nanocrystals according to claim 7, wherein isolating the metal chalcogenide nanocrystals comprises admixing a thiol and/or a phosphine with the nanocrystals; thereby forming a quenched mixture; and then extracting the nanocrystals from the quenched mixture.

13. The method for manufacturing colloidal metal chalcogenide nanocrystals according to claim 7 further comprising the step of maintaining the mixture at a temperature ranging from 60 C. to 130 C. during a predetermined duration ranging from 1 to 60 minutes after injection of the chalcogenide precursor.

14. The method for manufacturing colloidal metal chalcogenide nanocrystals according to claim 7, wherein said multiple inorganic ligands comprise at least one inorganic ligand selected from S.sup.2, HS.sup., Se.sup.2, Te.sup.2, OH.sup., BF.sub.4.sup., PF.sub.6.sup., Cl.sup., Br.sup., I.sup., As.sub.2Se.sub.3, Sb.sub.2S.sub.3, Sb.sub.2Te.sub.3, Sb.sub.2Se.sub.3, As.sub.2S.sub.3 or a mixture thereof.

15. A photoconductor, photodiode or phototransistor comprising: a photoabsorptive layer comprising a plurality of metal chalcogenide nanocrystals according to claim 1; and a first plurality of electrical connections bridging the photoabsorptive layer; wherein the plurality of metal chalcogenide nanocrystals are positioned such that there is an increased conductivity between the electrical connections and across the photoabsorptive layer, in response to illumination of the photoabsortive layer with light at a wavelength ranging from 3 m to 50 m; the carrier mobility is not less than 1 cm.sup.2V.sup.1s.sup.1.

16. The photoconductor, photodiode or phototransistor according to claim 15, wherein the photoabsorptive layer has a thickness ranging from 3 nm to 1 mm.

17. The photoconductor, photodiode or phototransistor according to claim 15, wherein the photoabsorptive layer has an area ranging from 100 nm.sup.2 to 1 m.sup.2.

18. A device comprising a plurality of photoconductors, photodiodes or phototransistors according to claim 15; and a readout circuit electrically connected to the plurality of photoconductors, photodiodes or phototransistors.

19. An IR-absorbing coating material comprising metal chalcogenide nanocrystals according to claim 1.

20. A bolometer or a pyrometer comprising metal chalcogenide nanocrystals according to claim 1.

21. The plurality of metal chalcogenide nanocrystals according to claim 1, wherein said plurality of metal chalcogenide nanocrystals exhibits an optical absorption feature in a range from 0.8 m to 25 m.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 represents an image of the transmission electron microscopy of small HgSe QD.

(2) FIG. 2 represents an image of the transmission electron microscopy of large HgSe QD.

(3) FIG. 3 represents an infrared absorption spectrum of small and large HgSe QD.

(4) FIG. 4 represents the absorbance spectrum of small and large HgSe QD of different size. The range of wavelength is selected to highlight the intraband transition.

(5) FIG. 5 represents the XRD diffractogramm of HgSe CQD.

(6) FIG. 6 represents the image of the large scale batch of small HgSe QD. More than 10 g of solid QD have been obtained with this synthesis.

(7) FIG. 7 ainfrared spectrum of HgSe CQD after the synthesis; bafter liquid phase ligand exchange.

(8) FIG. 7bis ainfrared spectrum of HgSe CQD after ligand exchange with different ligands; bpopulation of the conduction band is state as a function of the surface dipole magnitude for different surface capping ligands.

(9) FIG. 8 is a scheme of a dual (bottom and electrolytic) gated transistor based on a thin HgSe CQD film.

(10) FIG. 9 represents the current as a function of voltage for a thin film of HgSe QD under different gate bias.

(11) FIG. 10 illustrates a. drain current as a function of gate voltage for a transistor where the channel is made of HgSe CQD capped with As.sub.2S.sub.3 ligand. The device is operated in air at room temperature under 10 mV of drain bias. b is an histogram of the mobility estimated for a large range of film made of the same material.

(12) FIG. 11 represents the noise current density in thin film of HgSe QD.

(13) FIG. 12 represents the photo response of a thin film of HgSe QD illuminated by a 1.5 m laser.

(14) FIG. 13 represents the image the scanning electron microscopy image of pixel made out of a thin film of HgSe QD.

EXAMPLES

(15) The present invention is further illustrated by the following examples.

Example 1: HgSe Nanocrystal Synthesis2 Step Synthesis

(16) In a 50 mL three necks flask, 2 g of mercury acetate (Hg(OAc).sub.2) and 80 mL of oleic acid are degassed at 85 C. under vacuum for 30 min. the obtained stock solution is transparent yellowish. 4 mL of this solution are mixed with 10 mL of oleylamine and degassed at 85 C. for 30 min. Meanwhile 1.58 g of Se powder is dissolved by sonication in 20 mL of trioctylphosphine (TOPSe). The final solution is clear and transparent. Under Ar at a temperature between 60 and 130 C., 1 mL of TOPSe is injected in the flask containing the Hg precursor. The mixture immediately turns dark; the reaction is performed for 30 s to 60 min. Then 1 mL of dodecanethiol is injected to quench the reaction and the flask quickly cooled down using fresh air flow. The content of the flask was split into 50 mL tube and ethanol is added to precipitate the nanoparticle. After centrifugation for 5 min at 5000 rpm, the clear supernatant is trashed and the pellet redissolved in 10 mL clear toluene. This cleaning procedure is repeated for a second time using ethanol as non-solvent and toluene as good solvent. The pellet is again redissolved in toluene and 3 mL of acetone is added before centrifuging the solution. The formed pellet is saved and dried under nitrogen flow before being redissolved in toluene. 5 mL of ethanol is added to the supernatant which is further centrifuged to form a second pellet. The latter is also dried and redissolved in toluene. Finally 20 mL of ethanol is used to precipitate the remaining nanocrystal into the supernatant and the third fraction is further processed like the first two ones. The obtained nanocrystals are less than 10 nm in size, see FIG. 1 and have optical feature ranging from 3 to 7 m (at the intraband exciton peak, see FIGS. 3 and 4). Their crystalline nature of the CQD is highlighted by the XRD pattern, see FIG. 5.

Example 2: HgS Nanocrystal Synthesis2 Step Synthesis

(17) In a 50 mL three necks flask, 2 g of mercury acetate and 80 mL of oleic acid are degassed at 85 C. under vacuum for 30 min. the obtained stock solution is transparent yellowish. 4 mL of this solution are mixed with 10 mL of oleylamine and degassed at 85 C. for 30 min. Meanwhile 11 mg of Sulfur powder are dissolved by sonication in 3 mL of oleylamine. The final solution is clear and orange. Under Ar at a temperature between 60 and 120 C., the sulfur solution is injected in the flask containing the Hg precursor. The mixture immediately turns dark; the reaction is performed for 30 s to 60 min. Then 1 mL of dodecanethiol is injected to quench the reaction and the flask quickly cooled down using fresh air flow. The content of the flask was split into 50 mL tube and ethanol is added to precipitate the nanoparticle. After centrifugation for 5 min at 5000 rpm, the clear supernatant is trashed and the pellet redissolved in 10 mL clear toluene. This cleaning procedure is repeated for a second time using ethanol as non-solvent and toluene as good solvent. The pellet is again redissolved in toluene and 3 mL of acetone is added before centrifuging the solution. The formed pellet is saved and dried under nitrogen flow before being redissolved in toluene. 5 mL of ethanol is added to the supernatant which is further centrifuged to form a second pellet. The latter is also dried and redissolved in toluene. Finally 20 mL of ethanol is used to precipitate the remaining nanocrystal into the supernatant and the third fraction is further processed like the first two ones.

Example 3: HgSe Nanocrystal SynthesisQuasi One Step

(18) In a 25 mL three neck flask, 0.1 g of mercury acetate, 4 mL oleic acid and 10 mL oleylamine are degassed under vacuum at 85 C. for 30 min. The solution is clear and yellowish. Under Ar at the same temperature, 0.3 mL of TOPSe (1 M) is quickly injected. The solution turns dark immediately. Then 1 mL of dodecanethiol is injected to quench the reaction and the flask quickly cooled down using fresh air flow. The content of the flask was split into 50 mL tube and ethanol is added to precipitate the nanoparticle. After centrifugation for 5 min at 5000 rpm, the clear supernatant is trashed and the pellet redissolved in 10 mL clear toluene. This cleaning procedure is repeated for a second time using ethanol as non-solvent and toluene as good solvent. The pellet is again redissolved in toluene and 3 mL of acetone is added before centrifuging the solution. The formed pellet is saved and dried under nitrogen flow before being redissolved in toluene. 5 mL of ethanol is added to the supernatant which is further centrifuged to form a second pellet. The latter is also dried and redissolved in toluene. Finally 20 mL of ethanol is used to precipitate the remaining nanocrystal into the supernatant and the third fraction is further processed like the first two ones.

Example 4: Large Scale Synthesis Small HgSe Nanocrystals

(19) In a 1 L automated reactor, 10 g of Hg(OAc).sub.2 is dissolved in 200 mL of oleic acid. The flask is degassed under vacuum for 15 min at 85 C. Then 0.5 L of oleylamine is added and the flask is further degassed at the same temperature. The atmosphere is switched to Ar and the temperature adjusted at 85 C. Meanwhile 40 mL of TOPSe (1 M) is prepared by sonicating 3.16 g of Se powder in 40 mL of trioctylphosphine (TOP). 32 mL of the TOPSe solution is quickly injected into the reactor and the whole pot turns dark. The reaction is continued for 15 min, before being quenched by addition of 5 ml of dodecantiol and 1 mL of TOP. The content of the flask is mixed with the same volume of methanol in a 2 L Erlenmeyer. The solution is then filtered. The obtained solid is further cleaned using hexane and methanol. More than 10.8 g of solid have been obtained (see FIG. 6). Final storage is done in toluene.

Example 5: Large HgSe Nanocrystals Synthesis

(20) For nanocrystal larger with optical feature below 1500 cm.sup.1 (see FIG. 2), the following procedure has been developed. In a 25 mL three neck flask, 100 mg of Hg(OAc).sub.2 is dissolved in 4 mL of oleic acid and 10 mL of oleylamine. The flask is degassed under vacuum for 30 min at 85 C. The atmosphere is switched to Ar and the temperature adjusted between 60 and 130 C. depending on the expected final nanocrystal size. Meanwhile 0.13 g of SeS.sub.2 is dissolved in 2 mL of oleylamine under sonication. The brown mixture is injected into the flask and the color turns dark. After 1 to 60 min, 1 mL of dodecanethiol is used to quench the reaction. The heating mantle is removed and the flask is cooled using a flow of fresh air. The nanocrystals are precipitated by addition of ethanol. After centrifugation the formed pellet is redissolved in toluene. The cleaning procedure is repeated two other times. Nanoparticles are stored in toluene. However due to their large size (20 nm) they have a limited colloidal stability. The obtained nanocrystals are more than 10 nm in size, see FIG. 2 and have optical feature ranging from 7 to 25 m (at the intraband exciton peak, see FIGS. 3 and 4).

(21) In spite of the use of a sulfur based precursor, we see no evidence of sulfur in the final compound. The X-ray diffraction present the same peak as the material obtained using the TOPSe as Se precursor (see FIG. 5). Moreover the EDS data leads to no or extremely limited (few %) sulfide content which is consistent with the thiol used as ligands. This result seems consistent with the fact that performing the synthesis of small HgSe CQD using TOPS instead of TOPSe leads to no reaction for the previously described condition.

Example 6: HgSeTe Nanocrystal Synthesis

(22) In a 50 mL three necks flask, 2 g of mercury acetate and 80 mL of oleic acid are degassed at 85 C. under vacuum for 30 min. the obtained stock solution is transparent yellowish. 8 mL of this solution are mixed with 20 mL of oleylamine and degassed at 85 C. for 30 min. Meanwhile 1.58 g of Se powder is dissolved by sonication in 20 mL of trioctylphosphine. The final solution is clear and transparent. 1.27 g of Tellurium (Te) powder is dissolved in the glove box in 10 mL of TOP and stirred for two days, the final TOPTe solution is yellow and clear. Under Ar at 85 C., 1 mL of TOPSe is injected in the flask containing the Hg precursor. The mixture immediately turns dark. Immediately we start injecting dropwise and over 30 min the TOPTe precursor. After 30 min, 2 mL of dodecanethiol is injected to quench the reaction and the flask quickly cooled down using fresh air flow. The content of the flask was split into 50 mL tube and ethanol is added to precipitate the nanoparticle. After centrifugation for 5 min at 5000 rpm, the clear supernatant is trashed and the pellet redissolved in 10 mL clear toluene. This cleaning procedure is repeated for a second time using ethanol as non-solvent and toluene as good solvent. The pellet is again redissolved in toluene and 3 mL of acetone is added before centrifuging the solution. The formed pellet is saved and dried under nitrogen flow before being redissolved in toluene. 5 mL of ethanol is added to the supernatant which is further centrifuged to form a second pellet. The latter is also dried and redissolved in toluene. Finally 20 mL of ethanol is used to precipitate the remaining nanocrystal into the supernatant and the third fraction is further processed like the first two ones.

Example 7: HgSeCdS Core Shell Structure

(23) To grow a CdS shell on HgSe core nanocrystal the following procedure is used. We mix 30 mg of Na.sub.2S in 2 ml of NMFA in a 4 mL vial up to dissolution. The core are then precipitated by addition of acetonitrile to remove the excess of sulfide and redispersed in NMFA. Then 500 l of 0.2 M cadmium acetate in NMFA are added in the vial. After the almost immediate reaction the excess of precursors is removed by precipitation of the nanocrystals with a mixture of toluene and acetonitrile (5:1). The solid obtained by centrifugation is redissolved in NMFA. The procedure is repeated 3.5 times.

Example 8: Solid State Ligand Exchange

(24) A film of HgSe nanocrystal capped with dodecanethiol ligand is deposited by dropcasting a solution of nanocrystal dispersed into a 9:1 hexane octane mixture. The film is then dipped for 30 s in a 1% in volume solution of ethanedithiol in ethanol. The film is then rinsed in pure ethanol.

Example 8bis: Solid State Ligand Exchange

(25) A film of HgSe nanocrystal capped with dodecanethiol ligand is deposited by dropcasting a solution of nanocrystal dispersed into a 9:1 hexane octane mixture at low concentration (0.1-0.5 mg/mL.sup.1). The film is then dipped for 60 s in a 0.5% in volume solution of ethanedithiol in ethanol. The film is then rinsed in pure ethanol and dried. The procedure is repeated 10 time to build a thin homogeneous film with a very limited amount of cracks.

Example 9: Solid State Ligand Exchange

(26) A film of HgSe nanocrystal capped with dodecanethiol ligand is deposited by dropcasting a solution of nanocrystal dispersed into a 9:1 hexane octane mixture. The film is then dipped for 30 s in a 1% in volume solution of NH.sub.4Cl in ethanol. The film is then rinsed in pure ethanol.

Example 10: Solid State Ligand Exchange

(27) A film of HgSe nanocrystal capped with dodecanethiol ligand is deposited by dropcasting a solution of nanocrystal dispersed into a 9:1 hexane octane mixture. Meanwhile As.sub.2S.sub.3 powder is mixed with short liquid amine such as propylamine or butylamine (at a 1 to 10 mg/mL concentration). The solution is sonicated to obtain clear yellow solution. This solution is diluted 10 times with acetonitrile. The nanocrystal film is dipped in this solution for 30 s and rinsed in pure ethanol. The film is finally dried using Ar flow.

Example 10bis: Solid State Ligand Exchange

(28) A film of HgSe nanocrystal capped with dodecanethiol ligand is deposited by dropcasting a solution of nanocrystal dispersed into a 9:1 hexane octane mixture. Meanwhile Na2S or NaSH solid is mixed with ethanol at 0.5% in weight. The solution is sonicated to obtain clear solution. The nanocrystal film is dipped in this solution for 30 s and rinsed in pure ethanol. The film is finally dried using Ar flow.

Example 11: Liquid Ligand Exchange

(29) A few mg of Na.sub.2S are dissolved in 2 mL of N-methylformamide. The solution is sonicated for 2 min. In a test tube 1 mL of the previous solution is introduced with 3 mL of HgSe QD dispersed in hexane. The solution is strongly stirred and further sonicated. A phase transfer of the nanoparticle occurred and the polar phase turns dark. The non-polar phase is then clean three times by adding hexane and let the solution settled. The clear top phase is trashed each time. Finally 3 mL of ethanol are added and the tube is centrifuged at 3000 rpm for 3 min. the liquid is trashed and the formed pellet is dried under nitrogen flow, before getting redispersed into fresh N methyl formamide.

Example 12: Liquid Ligand Exchange with As.SUB.2.S.SUB.3

(30) A few mg of As.sub.2S.sub.3 are dissolved into 1 mL of propylamine. The solution is sonicated for 1 min. The final solution is yellow and clear. 500 L of this solution is then mixed with 1 mL of N methyl formamide. The solution is sonicated for 2 min. In a test tube 1 mL of the previous solution is introduced with 3 mL of HgSe QD dispersed in hexane. The solution is strongly stirred and further sonicated. A phase transfer of the nanoparticle occurred and the polar phase turns dark. The non-polar phase is then clean three times by adding hexane and let the solution settled. The clear top phase is trashed each time. Finally 3 mL of ethanol are added and the tube is centrifuged at 3000 rpm for 3 min. the liquid is trashed and the formed pellet is dried under nitrogen flow, before getting redispersed into fresh N methyl formamide. The FIG. 7 shows how the infrared spectrum of HgSe CQD is affected by the ligand exchange process.

Example 13: Liquid Ligand Exchange with Sb.SUB.2.S.SUB.3

(31) A few mg of Sb.sub.2S.sub.3 are dissolved into 1 mL of ethylenediamine by stirring the solution for 24 h at room temperature. n. The final solution is white and bit turbid. 500 L of this solution is then mixed with 1 mL of N methyl formamide. The solution is sonicated for 2 min. In a test tube 1 mL of the previous solution is introduced with 3 mL of HgSe QD dispersed in hexane. The solution is strongly stirred and further sonicated while heating gently the solution with a heat gun. A phase transfer of the nanoparticle occurred and the polar phase turns dark. The non-polar phase is then clean three times by adding hexane and let the solution settled. The clear top phase is trashed each time. Finally 3 mL of ethanol are added and the tube is centrifuged at 3000 rpm for 3 min. the liquid is trashed and the formed pellet is dried under nitrogen flow, before getting redispersed into fresh N methyl formamide.

Example 14: Atomic Layer Deposition (ALD) Encapsulation

(32) A film of HgSe nanocrystal capped with dodecanethiol ligand is deposited by dropcasting a solution of nanocrystal dispersed into a 9:1 hexane octane mixture. The film is then dipped for 30 s in a 1% in volume solution of ammonia in ethanol. The film is then rinsed in pure ethanol. Then the film is introduced in the ALD setup and put under primary vacuum. The film is then sequentially exposed to flow of diethylzinc and water. Each exposition is followed by a waiting step of at least 1 min. 10 layers of the ZnO are deposited and the film is finally cooked at 70 C. for 10 min.

Example 15: Electrodes Fabrication

(33) A n type doped silicon wafer with a 400 nm SiO.sub.2 top layer, is cut into pieces of 11 cm2. The substrate is then rinsed by sonication into acetone for 5 min and further rinsed under isopropanol flow. Then the substrate is processed for 5 min under O.sub.2 plasma. Using spin coating we deposit photosensitive resist AZ5214. The resist is further cooked under hot plate at 110 C. for 90 s. Using UV lithography mask the electrodes pattern is illuminated for 2 s. A second bake of the wafer on hot plate at 125 C. is conducted for 2 min. Then a UV flood exposure is operated for 40 s. Finally the resist is developed by dipping the film for 32 s into AZ326 developer. Then 3 nm of Cr and 40 nm of gold are deposited thanks to a thermal evaporator. Finally the lift off is conducted by dipping the substrate into acetone for 1 h, before rinsing the electrodes with isopropanol.

Example 16: Transparent and Flexible Electrodes Fabrication

(34) Indium tin oxide (ITO) coated on polyethylene terephthalate (PET) (80 nm coating with a 60 /cm.sup.2 resistance) sheet are purchased from Sigma-Aldrich. The film is rinsed using acetone and then isopropanol before being dried. AZ 5214E resist is spin-coated and then baked for 90 s on a 110 C. hot-plate. The film is then exposed to UV for 4 s though a shadow mask. The resist is then developed for 45 s in AZ 726 and rinsed in pure water. The naked ITO is then etched using 25% HCl solution for 15 s and then quickly rinsed in pure water. The lift-off of the resist is made by dipping the substrate in acetone for 5 min and then rinsing the film with isopropanol. The designed electrodes are interdigitated electrodes with 50 m spacing. Each electrode is itself 50 m large and 1 mm long. The total active area is 1 mm2. A side gate electrode is also present for electrolyte gating.

Example 17: Nanotrench Fabrication

(35) On a Si/SiO.sub.2 wafer, a first electrode is prepared either using standard optical lithography or electron beam lithography. In a typical preparation AZ 5214 E resist is deposit by spin coating on the wafer. The wafer is then baked for 90 s at 110 C. A first UV exposure using the lithography mask is performed for a couple second. Then the film is further bake at 125 C. for 2 minutes. We then process to metal deposition by evaporating Ti (5 nm) and a layer of gold (54 nm) using electron evaporator. Finally the lift off is conducted by dipping the substrate into acetone for 12 h, before rinsing the electrodes with isopropanol. A second pattern is prepared using the same lithography procedure. The second metallic evaporation is made while the sample is tilted by 60 C. in order that the first electrode shadows some part of the second pattern. In this case 5 nm of Cr and 50 nm of gold are deposited. This shadow effect allows the formation of nanogap at the scale of a few tens nanometers.

Example 18: Photodetector with a Unipolar Barrier

(36) On a Si/SiO.sub.2 wafer two electrodes were designed, typically 2 mm long and spaced by 20 m. Then one electrode is connected to a high bias DC source. The substrate face a 1 cm.sup.2 glass slide coated with an indium tin oxide (ITO). This second electrode is connected to the negative side of the DC high bias voltage supply. The substrate functionalized with the two electrodes and the ITO coated electrode is dipped in a solution of CdTe NPL. A 400V voltage is applied for 30 s. We observe deposition of CdTe on the positive electrode. The substrate with the two electrodes is now dipped into a solution of Na.sub.2S in ethanol for 1 min, before being rinsed in pure ethanol. The electrodes are finally dried under air flow. The electrodes are then annealed on a hot plate for 1 h at 300 C. Then HgSe nanoparticle capped with As.sub.2S.sub.3 are dropcasted on the functionalized electrodes and heated on a hot plate in a glove box for 10 min at 100 C.

Example 19: Photoconductive Device Fabrication

(37) In the glove box, the HgSe CQD capped with As.sub.2S.sub.3 is dropcasted on the electrode on a hot plate at 100 C.

Example 20: Electrolyte Fabrication

(38) 50 mg of LiClO.sub.4 are mixed with 230 mg of PEG on a hot plate in an Ar filled glove box at 170 C. for 2 h.

Example 21: Back Gated Transistor Fabrication

(39) In the glove box, the HgSe CQD capped with As.sub.2S.sub.3 is dropcasted on the electrode on a hot plate at 100 C. Once the film is dried, we brush pure polyethylene glycol as protective layer.

Example 22: Dual Gated Transistor Fabrication

(40) In the glove box, the HgSe CQD capped with As.sub.2S.sub.3 is dropcasted on the electrode on a hot plate at 100 C. Meanwhile the electrolyte is softened at 100 C. The melted electrolyte is now clear and is brushed on the CQD film. A copper grid is then deposited on the top of the electrolyte and can be used as top gate. A scheme of the device is shown on FIG. 8. Current as a function of the applied voltage of the device are given on FIG. 9. Current as a function of the applied gate bias of the device are given on FIG. 10. The noise of the device is plotted on FIG. 11. The photoresponse of the system as a function of the frequency is given on FIG. 12.

Example 23: Lithography to Design Pixel

(41) Films of HgSe CQD capped with As.sub.2S.sub.3 are dropcasted on a clean Doped Si wafer. The film is typically 100 nm thick. In the clean room, PMMA is spin coated and baked at 160 C. for 15 min. A 6.4 nA current and 20 kV electron acceleration is used to perform the e-beam writing. The film is developed using a Methyl isobutyl ketone (MIBK):Isopropanol (IPA) mixture and rinsed in pure isopropanol. The etching of the nanocrystal film result from an O.sub.2 plasma operated for 5 min. Finally the resist is removed by dipping the film for 5 min in pure acetone. The film is further rinsed in pure IPA and dried. This method allows the design array of pixel with a 20 m and 60 m pitch, see FIG. 13).