Nanorod thin-film transitors

Abstract

A method for forming an electronic switching device on a substrate, wherein the method comprises depositing the active semiconducting layer of the electronic switching device onto the substrate from a liquid dispersion of ligand-modified colloidal nanorods, and subsequently immersing the substrate into a growth solution to increase the diameter and/or length of the nanorods on the substrate, and wherein the as-deposited nanorods are aligned such that their long-axis is aligned preferentially in the plane of current flow in the electronic switching device.

1 . An electronic switching device having a semiconducting layer that comprises inorganic semiconducting elongate nanoparticles having a longer dimension and a shorter dimension, the average ratio of the length of the longer dimension to the length of the shorter dimension for the nanoparticles of the layer being in the range 2 to 50 and the average length of the longer dimension of the nanoparticles of the layer being less than 1000 nm, wherein the nanoparticles of the layer are generally mutually aligned. 
     
     
         2 . The electronic switching device of  claim 1 , wherein the electronic switching device further comprises first and second contacts defining a current transport path through the semiconducting layer extending therebetween, the nanoparticles being generally aligned along the direction of the current transport path. 
     
     
         3 . The electronic switching device of  claim 2 , wherein the distance between the first and second contacts defines a channel length and the ratio of the channel length to the average length of the longer dimension of the nanoparticles of the layer is larger than 1. 
     
     
         4 . The electronic switching device of  claim 1 , wherein the nanoparticles are uniaxially aligned within one or more domains having a diameter greater than 1000 nm. 
     
     
         5 . The electronic switching device of  claim 1 , wherein at least some of the nanoparticles are fused together. 
     
     
         6 . The electronic switching device of  claim 1 , wherein the nanoparticles comprise an oxide semiconductor. 
     
     
         7 . The electronic switching device of  claim 6 , wherein said oxide semiconductor is zinc oxide, tin oxide, zinc tin oxide, indium oxide, zinc indium oxide or indium gallium zinc oxide. 
     
     
         8 . The electronic switching device of  claim 1 , wherein the mobility of the semiconducting layer is at least 0.5 cm 2 V −1 s −1 . 
     
     
         9 . The electronic switching device of  claim 1 , wherein the density of the film is at least 50% of the bulk density of the inorganic material of which the nanoparticles comprise. 
     
     
         10 . A method for fabricating a film of nanoparticles on a substrate, the method comprising:
 forming a dispersion of elongate inorganic nanoparticles in a solvent, the nanoparticles having one or more ligand molecules attached to their surface, the nanoparticles having a longer dimension and a shorter dimension, and the ligand molecules including a functional group that enhances the stability of the dispersion of the nanoparticles in the solvent; and   causing the nanoparticles to be deposited onto the substrate from the dispersion by removal of the solvent at a surface of the dispersion.   
     
     
         11 . The method of  claim 10 , wherein the ligands are organic or partly organic. 
     
     
         12 . The method of  claim 11 , wherein said semiconducting nanoparticles comprise an oxide semiconductor. 
     
     
         13 . The method of  claim 12 , wherein said oxide semiconductor is zinc oxide, tin oxide, zinc tin oxide, indium oxide, zinc indium oxide or indium gallium zinc oxide. 
     
     
         14 . The method of  claim 10 , wherein the ligands are selected so as to cause the concentration of nanoparticles to be higher at the surface of the solution than in the bulk of the solution, and the shape of the nanoparticles is selected so as to promote mutual alignment of the nanoparticles. 
     
     
         15 . The method of  claim 10 , wherein the ratio of the length of the longer dimension to the length of the shorter dimension for the nanoparticles in solution is in the range 2 to 50 and the average length of the longer dimension of the nanoparticles in solution is less than 1000 nm. 
     
     
         16 . The method of  claim 10 , wherein the density of the film is at least 50% of the bulk density of the inorganic material of which the nanoparticles are comprised. 
     
     
         17 . The method of  claim 10 , wherein the solvent is removed at a surface of the solution in such a way so as to define a direction of preferential orientation for the nanoparticles and cause the nanoparticles to become at least partially aligned along that direction. 
     
     
         18 . The method of  claim 17 , wherein the film defines a geometric plane and the direction is out of the geometric plane defined by the film. 
     
     
         19 . The method of  claim 17 , wherein the film defines a geometric plane and the direction lies in the geometric plane defined by the film. 
     
     
         20 . The method of  claim 17 , wherein the direction of preferential orientation is defined by the flow of solvent during removal of the solvent at a surface of the solution. 
     
     
         21 . The method of  claim 10 , wherein the concentration of the nanoparticles in the solvent is at least 5 mg/ml. 
     
     
         22 . The method of  claim 10 , wherein the solution has a lower surface tension than the pure solvent due to the presence of the ligands in the solution. 
     
     
         23 . The method of  claim 10 , wherein the ligands are one or more of octylamine, butylamine, hexylamine, and any other alkylamine. 
     
     
         24 . The method of  claim 10 , wherein the ligands are bound to the surface of the nanoparticles by a chelating bond. 
     
     
         25 . The method of  claim 10 , wherein the solvent comprises a mixture of solvents. 
     
     
         26 . The method of  claim 25 , wherein said mixture comprises a polar and a nonpolar solvent. 
     
     
         27 . The method of  claim 26 , where said mixture comprises an alcohol and an organic solvent. 
     
     
         28 . The method of  claim 10 , wherein, subsequent to causing the nanoparticles to be deposited from solution, the film is heated so as to remove the ligands. 
     
     
         29 . The method of  claim 28 , wherein said removal of ligands occurs by heating the film at a temperature less than 250° C. 
     
     
         30 . The method of  claim 28 , wherein said heating step is induced by thermal annealing or by irradiation with light absorbed by the nanoparticles. 
     
     
         31 . The method of  claim 28 , wherein, subsequent to removing the ligands, the film is immersed in a growth solution of nanoparticles in a solvent. 
     
     
         32 . The method of  claim 31 , wherein the growth solution is a hydrothermal growth solution. 
     
     
         33 . The method of  claim 32 , wherein the hydrothermal growth solution is an aqueous solution comprising zinc nitrate and ethylenediamine. 
     
     
         34 . The method of  claim 31 , wherein the growth solution is heated at a temperature below the bulk melting point of the nanoparticle material so as to cause at least some of the nanoparticles to fuse together. 
     
     
         35 . The method of  claim 34 , wherein the temperature to which the growth solution is heated is less than 100° C. 
     
     
         36 . The method of  claim 31 , wherein, subsequent to immersing the film in a growth solution, the film is heated so as to cause annealing of the nanoparticle film. 
     
     
         37 . The method of  claim 36 , wherein the film is heated under an atmosphere predominantly comprising nitrogen and hydrogen gases. 
     
     
         38 . The method of  claim 36 , wherein the annealing temperature to which the film is heated is less than 250° C. 
     
     
         39 . The method of  claim 10 , wherein the nanoparticles are deposited as a continuous film by means of one of spin coating, drop coating, blade coating and microgravure coating, or as a patterned but locally continuous film by direct printing. 
     
     
         40 . The method of  claim 10 , wherein the nanoparticles are semiconducting. 
     
     
         41 . The method of  claim 40 , wherein said film of nanoparticles forms part of the active layer of an electronic device. 
     
     
         42 . The method of  claim 41 , wherein said electronic device further comprises first and second contacts defining a current transport path through the semiconducting layer extending therebetween, the nanoparticles being generally aligned along the direction of the current transport path. 
     
     
         43 . The method of  claim 41 , wherein said electronic device is an electronic switching device. 
     
     
         44 . The method of  claim 41 , wherein said electronic device is diode, such as a light-emitting, light-sensing or photovoltaic diode. 
     
     
         45 . The method of  claim 42 , wherein said elongate nanoparticles are aligned with their long dimension preferentially oriented in the plane of the substrate. 
     
     
         46 . The method of  claim 42 , wherein said elongate nanoparticles are aligned with their long dimension preferentially oriented normal to the plane of the substrate. 
     
     
         47 . The method of  claim 41 , wherein the mobility of the active semiconducting layer is at least 0.5 cm 2 V −1 s −1 . 
     
     
         48 . The method of  claim 42 , wherein the distance between the first and second contacts defines a channel length and the ratio of the channel length to the average length of the longer dimension of the nanoparticles of the layer is larger than 1. 
     
     
         49 . The method of  claim 10 , wherein the removal of the solvent occurs when the dispersion is in contact with the substrate. 
     
     
         50 . A method for fabricating a film of nanoparticles, the method comprising:
 forming a dispersion of elongate nanoparticles in a solvent, the nanoparticles having a longer dimension and a shorter dimension and having one or more ligand molecules attached to their surface; and   causing the nanoparticles to be deposited from the dispersion by removal of the solvent at a surface of the dispersion;   wherein the ligand molecules are selected so as to cause the concentration of nanoparticles to be higher at the surface of the solution than in the bulk of the solution, and the shape of the nanoparticles is selected so as to promote mutual alignment of the nanoparticles.   
     
     
         51 . An active semiconducting layer that comprises inorganic semiconducting elongate nanoparticles having a longer dimension and a shorter dimension, the average ratio of the length of the longer dimension to the length of the shorter dimension for the nanoparticles of the layer being in the range 2 to 50 and the average length of the longer dimension of the nanoparticles of the layer being less than 1000 nm, wherein the nanoparticles of the layer are generally mutually aligned. 
     
     
         52 . The active semiconducting layer of  claim 51 , wherein the nanoparticles are uniaxially aligned within domains having a diameter greater than 1000 nm. 
     
     
         53 . The active semiconducting layer of  claim 51 , wherein at least some of the nanoparticles are fused together. 
     
     
         54 . The active semiconducting layer of  claim 51 , wherein the nanoparticles are zinc oxide nanoparticles. 
     
     
         55 . The active semiconducting layer of  claim 51 , wherein the mobility of the semiconducting layer is at least 0.5 cm 2 V −1 s −1 .