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  • C01G39/00—Compounds of molybdenum
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
  • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
  • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
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  • C01—INORGANIC CHEMISTRY
  • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
  • C01G39/00—Compounds of molybdenum
  • C01G39/02—Oxides; Hydroxides
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
  • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
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  • C01P2002/80—Crystal-structural characteristics defined by measured data other than those specified in group C01P2002/70
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  • C01P2004/00—Particle morphology
  • C01P2004/01—Particle morphology depicted by an image
  • C01P2004/03—Particle morphology depicted by an image obtained by SEM
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
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  • C01P2004/00—Particle morphology
  • C01P2004/01—Particle morphology depicted by an image
  • C01P2004/04—Particle morphology depicted by an image obtained by TEM, STEM, STM or AFM
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
  • C01P2004/00—Particle morphology
  • C01P2004/10—Particle morphology extending in one dimension, e.g. needle-like
  • C01P2004/16—Nanowires or nanorods, i.e. solid nanofibres with two nearly equal dimensions between 1-100 nanometer

Definitions

• This invention relates to the development of a molybdenum oxide phototransistor and its method of synthesis, in particular, to a potassium-intercalated molybdenum oxide phototransistor.
• Molybdenum oxide and its derivatives have been a subject of increasing research interests due to their broad technological applications, such as electrochromic devices, batteries, photochromic devices, field emission devices, and gas sensors.
• 35"40 Bulk Mo0 3 exhibits a layered structure, which is well suited for intercalation of ionic species, such as Li+, to achieve novel physical and chemical properties.
• 41 ,42 The intercalation becomes more facile in nanostructures than in bulk due to the high surface-to-volume ratio of the nanostructures, which provides large contact surface areas for ion insertion, high flexibility, and adequate toughness for accommodating strains induced by ion insertion.
• the self-diffusion method can be utilized to prepare intercalated Mo0 3 nanostructures by immersing Mo0 3 nanobelts in LiCI solution.
• the efforts to intercalate large ions such as K+ into the Mo0 3 nanostructure without damaging the integrity of the well-aligned layered structure has essentially never been successful due to the large size of these ions compared to the size of the gap between layers.
• the attempt to intercalate K+ ions in galvanostatic mode using the standard electrode configuration has failed to maintain the layered structure; instead, transformation from the crystalline structure to an amorphous structure occurred.
• Photoconductivity is one of the most studied phenomena in nanowires (NWs) mainly due to their large surface to volume ratio, nanosize spatial constraints and quantum confinement. They facilitate the applications of the NWs as photodetectors, 1 ,2 photovoltaics, 3"5 optical switchs, 6 and optical interconnects.
• NWs nanowires
• phototransistor is one of the basic building blocks for nanoelectronic circuits.
• Molybdenum oxide (Mo0 3 ) a wide-band-gap (3.2ev) n- type semiconductor, has been drawing increasingly attention in field emission devices (FED), 16,17 photodetectors , batteries, 18,19 catalysts, 20 sensors, 21 ,22 photochromic and electrochromic materials. 23,24 Like other metal oxide nanowires, the main applications of Mo0 3 in photoelectronics are limited by its wide bandgap. Its low electrical n-type conductivity (the resistivity is of the order of 10 10 ⁇ ) always inhibits its practical implementation as well. 25 Impurity doping is one of the most common way used to modify the electrical properties of the material and one advantage about Mo0 3 is its rich intercalation chemistry made possible by its layered structure. However, due to the size limitation of the gap between layers, only small ions, such as lithium has been successfully intercalated through immersing Mo0 3 nanostructure in LiCI solution 6
• the present invention demonstrates the feasibility in intercalating large ions such as potassium without damaging the integrity of the layered structure of Mo0 3 .
• a surprisingly simple procedure is developed to synthesize potassium-intercalated Mo0 3 nanobundles with the integrity of the layered structure remaining intact. While the material displays semiconductor-like behavior, dramatic enhancement of the electric conductivity from 10 "6 S rrf 1 of Mo0 3 to 24 S m " upon potassium uptake was observed. Density functional theory calculations were performed to assist in structural determination and to elucidate the electronic property of the nanobundles. It was found that the K atoms occupy the oxygen vacancy sites in the lattice.
• a method of synthesizing a molybdenum oxide phototransistor comprising: placing a foil of molybdenum in a furnace; placing a sheet of muscovite (K(Al2)(Si 3 AI)Oi 0 (OH) 2 ) at a predetermined distance from the foil of molybdenum in the furnace; controlling airflow into the furnace to provide sufficient oxygen for crystalline growth of potassium-intercalated molybdenum oxide (K x Mo0 3 ) while maintaining a preset temperature of the furnace for a time period to produce the K x Mo0 3 nanowire; transferring the K x Mo0 3 nanowire onto a substrate; and forming a covering of electrodes on the nanowire to produce the molybdenum oxide phototransistor.
• the preset temperature may range from about 300 °C to about 900 °C.
• the time period may range from about 10 minutes to about 12 hours.
• the predetermined distance may be about 1 mm.
• Forming the covering of electrodes may comprise using photolithography to form a design of metal finger electrodes.
• the method may further comprise thermal evaporating aluminium to form the metal finger electrodes.
• the crystalline growth may comprise growth from grain boundaries via thermal evaporation to incorporate K atoms from the muscovite into a Mo0 3 lattice.
• the crystalline growth may be in a [001] direction.
• a molybdenum oxide phototransistor comprising: a substrate; a potassium-intercalated molybdenum oxide (K x Mo0 3 ) nanowire on the substrate; and a covering of electrodes on the nanowire.
• the electrodes may be made of aluminium.
• the molybdenum oxide phototransistor may be synthesized according to the method of the first aspect.
• the K x Mo0 3 nanowire may have an intact layered structure comprising K atoms incorporated into a Mo0 3 lattice.
• the substrate may comprise a layer of Si0 2 on a layer of Si.
• the substrate may further comprise a dielectric coating of Si 3 N 4 .
• the substrate may comprise a bendable plastic.
• a method of synthesizing a potassium- intercalated molybdenum oxide (K x Mo0 3 ) nanowire comprising: placing a foil of molybdenum in a furnace; placing a sheet of muscovite (K(Al2)(Si 3 AI)O 10 (OH) 2 ) at a predetermined distance from the foil of molybdenum in the furnace; controlling airflow into the furnace to provide sufficient oxygen for crystalline growth of potassium-intercalated molybdenum oxide (K x Mo0 3 ) while maintaining a preset temperature of the furnace for a time period to produce the K x Mo0 3 nanowire.
• the preset temperature may range from about 300 °C to about 900 °C.
• the time period may range from about 10 minutes to about 12 hours.
• the predetermined distance may be about 1 mm.
• the crystalline growth may comprise growth from grain boundaries via thermal evaporation to incorporate K atoms from the muscovite into a Mo0 3 lattice.
• the crystalline growth may be in a [001] direction.
• FIG. 1 (a) is an SEM image of an individual nanowire; upper right inset shows the EDX spectrum of the nanowire and bottom left inset shows the low magnification TEM image;
• FIG. 1 (b) is a Raman spectra of a K-intercalated o0 3 nanowire (grey curve) and pure
• FIG. 1 (c) is an SAED pattern of K x Mo0 3 on a (010) surface
• FIG. 1 (d) is an SEM image of an individual nanowire device
• FIG. 2 (a) is a typical I D S-VDS curve of a single nanowire device in dark field, insets show a schematic symbol of potential applications (diode and phototransistor);
• FIG. 2 (b) is an EDX spectra measured at three different spots along a nanowire growth direction;
• FIG. 2 (c) is a typical l D s-V D s curve (curve labeled as 1 mW/cm 2 ) of a single nanowire device in white light illumination, inset is a close up of the dark field curve in V DS ⁇ 0 region;
• FIG. 2 (d) is a graph of output current plotted as a function of light power intensity
• FIG. 2 (e) is a graph of l D s-V D s characteristics of a device under different power illumination at a VDS ⁇ 0 region;
• FIG. 2 (f) is a close up of characteristics at a region of -4.5V ⁇ V DS ⁇ 0;
• FIG. 2 (g) is a graph of l DS -V D s characteristics of a device under different power illumination at a V DS ⁇ 0 region;
• FIG. 2 (h) is a close up of characteristics at a region of 0 ⁇ V DS ⁇ 4.5V;
• FIG. 3 is a schematic illustration of components of an entire output characteristic of a device
• FIG. 4 (b) is a graph of typical l D s-V D s characteristics of a device at different temperatures
• FIG. 5 (a) is a graph of photoresponse characteristics of a K*Mo0 3 phototransistor at different optical powers
• FIG. 5 (b) is a graph of photoswitching rate test of the K x Mo0 3 phototransistor mentioned in
• FIG. 5 (c) is a graph showing that the K x Mo0 3 phototransistor mentioned in FIG. 5(a) is able to respond to all light wavelengths from 488nm to 1 100 nm;
• FIG. 5 (d) is a graph of l D s-Time of the K x Mo0 3 phototransistor mentioned in FIG. 5(a) showing consistent device performance;
• FIG. 6 (a) is a schematic representation of a synthesis system
• FIG. 6 (b) shows a typical morphology of a single K x Mo0 3 nanobundle, inset image is a close up of the right end of the ⁇ ⁇ ⁇ 0 3 nanobundle;
• FIG. 6. is an electron diffraction pattern of a Mo0 3 microbelt on a (010) surface; the highlighted rectangle denotes the orthorhombic lattice structure, the inset image shows a SEM image of a typical Mo0 3 microbelt growing in a [001 ] direction;
• FIG. 6 (d) is an electron diffraction pattern of a K Mo0 3 nanobundle on a (010) surface; the highlighted rectangle formed by large bright spots represents a lattice structure of the K-intercalated Mo0 3 , inset image shows a TEM image of a typical K x Mo0 3 nanobundle growing in a [001 ] direction;
• FIG. 7 shows XRD patterns of a mica substrate with Mo0 3 microbelts (upper chart) and a mica substrate with both Mo0 3 microbelts and K x Mo0 3 nanobundles (lower chart), label peaks with M are muscovite peaks while label peaks without notation are Mo0 3 peaks, the three peaks that are labeled with asterisks denote a layered structure of K x Mo0 3 correspond to expand along (020), (040), and (060), the rest of the peaks could be attributed to other faces of K x Mo0 3 ;
• FIG. 8 (a) is a schematic illustration of optimized structure of pure Mo0 3 ;
• FIG. 8 (b) is a schematic illustration of optimized structure of pure Mo0 3 with K as intercalants
• FIG. 8 (c) is a schematic illustration of optimized structure of pure Mo0 3 with K as occupants
• FIG. 8 (d) is a schematic illustration of optimized structure of a mixed case, where the pink
• FIG. 9 is an l-V curve of individual K x Mo0 3 nanobundle in different temperatures, the inset figures show schematic view and SEM image of K x Mo0 3 nanobundle contacted by electrodes;
• FIG. 10 is an XPS spectrum of Mo-3d peaks in K x Mo0 3 nanobundle
• FIG. 11 is a graph of calculated band structure and density of states (DOS) of K x Mo0 3 ;
• FIG. 12 is a schematic illustration of an ultrasensitive phototransistor fabricated based on K- intercalated Mo0 3 single nanowire;
• FIG. 13 (a) and (b) show similar results for a device alternatively made on a substrate of
• FIG. 14 is a spectrum of a light source
• FIG. 15a is a typical IDS-VDS curve of a representative single nanowire device in white light illumination
• FIG. 15b is the photoresponse characteristics of the device of FIG. 15a;
• FIG. 16 is long time trace of the photoresponse of the device of FIG. 15a;
• FIG. 17 shows the photoresponse characteristics of the device of FIG. 15a under different filters with wavelengths selected centred at (a) 488 nm, (b) 500 nm, (c) 514 nm, (d) 532 nm, (e) 550 nm, (f) 570 nm, (g) 600 nm, (h) 633 nm with light power density kept at 2.1 x10-2 mW/cm2 after employing the filters;
• FIG. 17i shows photocurrent as a function of wavelength
• FIG. 18 shows the photoresponse of the device of FIG. 15a under (a) 325 nm, (b) 532 nm, (c)
• Synthesis of K-intercalated Mo0 3 nanowire employed a simple and facile one-step vapor deposition method. Briefly, a piece of molybdenum foil was cleaned and loaded into the center of a horizontal tube furnace. A muscovite mica sheet (K(AI 2 )(Si 3 AI)Oi 0 (OH) 2 ) was placed on the top of the molybdenum foil with a gap of ca. 1 mm. The mica sheet not only acted as the substrate, it also provided the source of potassium. The system was ramped to 600 °C and dwelled for 6 hours with controlled air flow into the chamber. Mo was evaporated from the surface of the foil and oxidized in air flow.
• the oxidized Mo vapor reacted with potassium squeezed out from the edges and grain boundaries found on the mica and then intercalated into the growing Mo0 3 nanowire simultaneously to form the quasi-one-dimensional nanostructure.
• the nanowire was transferred to a piece of silicon and the morphology of the product was identified by scanning electron microscopy (SEM).
• FIG. 1 a A typical SEM image of a single nanowire is shown in FIG. 1 a.
• the nanowire displays a needle-like shape with the diameter reduces slightly along the growth direction from the bottom (upper left of FIG. 1 a) to the top (lower right of FIG. 1 a), the measured average diameter is about 800 nm with the length can be more than 200 ⁇ .
• Energy-dispersive spectroscopy (EDX) was carried out on a randomly selected spot of the nanowire and the representative spectrum is as shown in the insert of FIG. 1 a. The potassium peaks are clearly demonstrated, which indicates that K atoms have been intercalated into Mo0 3 successfully.
• We denoted the K-intercalated Mo0 3 nanowire as K x Mo0 3 .
• the ratio of K/Mo revealed by the EDX spectrum is 23:77.
• a diode laser (centered at 532 nm) was employed as the excitation source for the micro-Raman (Renishaw inVia) characterization.
• the Raman spectra of pure Mo0 3 (black line) and K x Mo0 3 nanowire (red line) are shown in FIG. 1 b.
• the Raman shift of these two spectra are completely different, which demonstrates the modification of the chemical bonds and lattice vabrations by potassium intercalation. 27
• These characterization results are highly suggestive of a diversification in electrical properties of K x Mo0 3 compared with pure Mo0 3 .
• FIG. 1 a illustrates a low magnification TEM image of a single wire, and the corresponding selected area electron diffraction (SAED) pattern of K Mo0 3 on the (010) surface is displayed in FIG. 1 c.
• SAED selected area electron diffraction
• a typical rectangular pattern consist of large bright spots is clearly shown with five weaker spots evenly distributed between two bright spots along one direction.
• the [001] direction is considered as the growth direction of K x Mo0 3 nanowire.
• the first type of material grows out from the flat surface of the mica substrate as large sized microbelts with a width of 3-5 pm, a length of 10-15 ⁇ , and a thickness of 1 pm. These microbelts were found to be the dominate product, as expected. However, at the grain boundaries of the mica substrate, growth of a new type of nanobundle with length around 200 pm extending out of the substrate was observed. Since the nanobundles were firmly attached to the substrate, only a segment of a nanobundle was transferred to Si substrate as shown in FIG. 6b with the length, width, and thickness of 87, 0.9, and 0.5 pm, respectively.
• the inset which displays the enlarged image of the right end of the nanobundle, indicates that the nanobundle is constructed by several parallel nanobelts. These nanobelts are of the same length as the nanobundle but much thinner with a width and a thickness of approximately 300 and 150 nm, respectively.
• the EDX spectrum elemental analysis on the two types of products reveals that the microbelts consist of pure Mo0 3 and the nanobundles contain a significant percentage of potassium atoms (denoted as K x Mo0 3 ).
• the K:Mo ratio in the K x Mo0 3 complex is fixed in the same nanobundle but differs slightly between different nanobundles with x ranging from 0.20 to 0.25.
• the atomic ratio of O over Mo in the K x Mo0 3 nanobundles is roughly 2.6 ⁇ 0.2, which is lower than the value in stoichiometric Mo0 3 compound, implying that O vacancies may exist.
• the grain boundaries in the mica layers allow the K atoms to be extracted to participate in the nanobundle growth.
• the surprisingly simple procedure for the synthesis of K x Mo0 3 nanobundles provides a highly effective approach to intercalate large ions into layered nanostructures.
• the Mo0 3 microbelts and K x Mo0 3 nanobundles were subsequently removed from the substrate and transferred to the TEM grids for further characterization.
• the selected area electron diffraction (SAED) pattern of the Mo0 3 microbelts on the (010) surface orientation is shown in FIG. 6c, and the inset image shows the SEM image of the Mo0 3 microbelts along the [001] growth direction.
• the microbelts exhibit a typical rectangular diffraction pattern on the (010) surface with a lattice adopting an orthorhombic configuration, similar to bulk Mo0 3 .
• the SAED pattern of the K x Mo0 3 nanobundles on the (010) surface is shown in FIG. 6d with the inset image displaying a low-magnification TEM image of the nanobundles along the [001] growth direction.
• the highlighted yellow rectangular diffraction pattern formed by large bright spots represents K-intercalated Mo0 3 structure on the (010) surface. Between two bright spots there are five weaker, evenly distributed spots along the [100] direction of the K x Mo0 3 nanobundles. These smaller diffraction spots suggest that K x Mo0 3 nanobundles possess a periodic superstructure with six primitive cells along the [100] direction. Elemental analysis on the mica grain boundaries upon the product removal indicates that Mo-K exchange occurs during the nanobundle growth.
• the growth of K x Mo0 3 nanobundles displays a strong orientational preference.
• the length of the nanobundles can grow as long as 200-300 Mm with a width of roughly 700-900 nm.
• the transferred nanobundles were placed on the TEM grid with the [010] direction perpendicular to the grid. Although the grid could be made to tilt by 15°, a clear diffraction pattern that contains information along the [010] direction could not be found. Instead, X-ray diffraction was utilized to further resolve the structure of the K x Mo0 3 nanobundles.
• the lattice constant b reported in Table 1 in the experiments is the value derived using the apex of the peaks in XRD spectrum from the most abundant nanobundles.
• the XRD spectrum of the K x Mo0 3 nanobundles clearly indicates that the complex preserves a layered structure as evidenced by the significant peaks located at (020), (040), and (060). This is distinctively different from the XRD analysis reported by Sian et al., 45 in which the intensity of all the peaks associated with Mo0 3 was reduced with the increase of the K contents and, in particular, all peaks vanished upon x reaching 0.3, indicating the complete loss of the initially layered structure.
• the device comprises a nanobundle on a substrate.
• the substrate comprises a layer of Si0 2 on a layer of Si.
• the lower inset panel of FIG. 9 displays the SEM image of an individual K x Mo0 3 nanobundle contacted by electrodes.
• the measured current is on the order of ca. 1 pA at ca. 5 V. From the measured effective length and cross section of this material, the electric conductivity of the Mo0 3 microbelt was estimated to be about 10 "6 S m "1 , consistent with the reported value of the Mo0 3 nanobelts.
• the measured current is 6.64 ⁇ at a bias of 5 V and the l-V curve displays typical semiconductor-like behavior.
• Further fieldeffect transistor (FET) measurement shows the K x Mo0 3 nanobundles exhibit n- type semiconductor behavior. It is remarkable that the electric conductivity is enhanced substantially by 7 orders of magnitude from 10 ⁇ 6 S m "1 of the Mo0 3 microbelts to 24 S rrr 1 . The magnitude is also 3 orders higher than that of the lithiated Mo0 3 bulk (Li 0 .
• the grey curve was fitted by Mo 6+ peaks (blue peaks 235.9 eV (Mo 3d 3/2) and 232.7 eV (Mo 3d 5/2)) and Mo 5+ peaks (green peaks 235.1 eV (Mo 3d 3/2) and 232.0 eV (Mo 3d 5/ 2)). 50
• the area ratio of Mo 5+ over Mo 6+ is around 1.5, suggesting that the valence of Mo is roughly +5.4. The result indicates that the Mo atoms are indeed partially reduced upon K insertion, consistent with the theoretical population analysis.
• the electronic structure of Mo0 3 is well understood, and the compound is an n-type semiconductor with a band gap of 3.3 eV. 51
• the valence band is largely dominated by the 2p orbitals of oxygen, while the conduction band consists of chiefly the 4d states of molybdenum with a significant contribution from the 2p states of oxygen.
• 52 Upon potassium uptake in the lattice, however, the electronic structure undergoes a substantial change due to the charge transfer from potassium to molybdenum, which forces electrons to populate the conduction band. This is clearly seen in the calculated band structure of the K x Mo0 3 lattice depicted in FIG. 11.
• the projected density of states (PDOS) for the K-4s and Mo-4d states indicates that the electrons from the K atoms are fully transferred to the adjacent Mo atoms. Because of the strong overlap between the Mo-4d orbitals and the 0-2p orbitals in the conduction band, in which the transferred electrons are populated and readily delocalized, the electric conductivity is thus significantly enhanced. Therefore, the conductivity enhancement arises solely from the reduced Mo atoms, which are aligned in the [001] direction as highlighted in FIG. 8. Electric conductivity along these rows thus reaches its maximum. Indeed, the calculated band structure displays wide bands across the Fermi level from G ⁇ B and Q ⁇ F.
• Density functional theory was used to assist the K x Mo0 3 structural determination and to understand the semiconductor-like behavior of the material. Our results suggest that the K atoms in the nanobundles most likely occupy the O vacancy sites, leading to considerable lattice relaxation due to the large size of potassium. This structural arrangement allows the K atoms to be intercalated without incurring large distortion of the M0O3 layered structure.
• the calculated band structure of the K0.25M0O2.75 indicates the K atoms are fully ionized, giving rise to the reduction of the adjacent Mo atoms. As a consequence, the conduction band is populated, leading to electron derealization along the rows containing low oxidation state Mo atoms in the [001] direction.
• the results are consistent with the measured high conductivity of the nanobundles and the observed variation of the conductivity with temperature.
• the novel properties of the K-enriched Mo0 3 nanobundles are envisaged to significantly enhance the performance of the electronic devices using compounds in the metal-intercalated M0O 3 family, and the simple preparation method opens a new opportunity to develop patterned nanostructured materials of large-ion- intercalated metal oxides.
• K x Mo0 3 nanobundles were synthesized by thermal evaporation method.
• a Mo foil (5 mm ⁇ 5 mm x 0.05 mm in size, from Aldrich Chemical Co., Inc.) was used as the Mo source and placed in ceramic boat, and a muscovite mica sheet (K 2 0-3AI 2 0 3 -6Si0 2 -2H 2 0, 8 mm 8 mm in size, from Alfa Aesar Co., Inc.) was placed 1 mm on top of the Mo foil as substrate and K source.
• the ceramic boat containing Mo foil and mica sheet was inserted into furnace (Carbolite MTF 12/25/250). The system was heated for 6 h in ambient at 600 °C, and a fan was used to blow fresh air into the furnace to provide enough oxygen for the growth.
• the nanobundles were characterized by a scanning electron microscope (SEM, JEOL JSM- 6700F), a transmission electron microscope (TEM, JEOL JEM-2010F) with built-in energydispersive spectroscopy (EDS), and X-ray diffraction (XRD, Philips X'Pert).
• SEM scanning electron microscope
• TEM transmission electron microscope
• EDS energydispersive spectroscopy
• XRD X-ray diffraction
• the single nanobundle device mentioned above was fabricated by transferring individual nanobundles from the growth substrate to Si0 2 /Si substrate and utilizing a photolithography method to achieve designed metal (Au(500 nm)/Cr(10 nm)) finger electrodes (of gap -10 ⁇ ) covering on nanobundle.
• the electrical measurements were carried out using Keithley 6430 source-measure unit.
• a single K x Mo0 3 nanowire was transferred to a silicon substrate coated with a 200 nm thick Si 3 N 4 dielectric for device fabrication and subsequent electrical characterization.
• a UV-laser lithography system (Heidelberg Instruments ⁇ 101 ) was employed to develop the device architecture. The fabrication was completed by thermal evaporating aluminum as the source- drain electrodes.
• the SEM image of an individual nanowire device with four electrodes is shown in FIG. 1 d. Both the electrode width and the separation between two electrodes are designed to be 10 ⁇ .
• the typical IDS-VDS curve is displayed in FIG. 2a, with V DS ramping from -10V to 10V. The blue curve indicates the dark current which was carried out without any light illumination.
• the output current of KJv1o0 3 nanowire exhibited an obvious unidirectional property.
• the current is minimal with the rising reverse bias while at forward range, output current increased dramatically with the voltage and almost a linear relationship was shown in the high voltage region.
• This curve is a typical l-V characteristic curve of a diode.
• the equivalent circuit is diagramed in the top inset of FIG. 2a, which indicates the current is only allowed to flow from positive to negative electrode.
• the unidirection of the output characteristic implies the exsitence of intrinsic barrier in the nanowire device.
• Further investigation through systematical EDX spectra along the K x Mo0 3 nanowire growth direction was carried out. And as labeled in FIG. 1 d, three representative spots were selected and the corresponding EDX results are shown in FIG. 2b.
• This property demonstrates the great potential of K Mo0 3 nanowire as a intrinsic barrier diode for nanoscale electronics application.
• K Mo0 3 nanowire also exhibits high sensitivity to light, even under weak light intensity.
• a halogen lamp with the main spectrum ranging from 400-1 100 nm (the spectrum of the lamp was captured using a spectrometer and result is shown in FIG. 14) was employed as white light to study the photo-response of the nanowire device during l-V measurement.
• the pink curve shown in FIG. 2c displays the typical IDS- D S curve under 1 mW/cm 2 of broad beam white light illumination.
• the output current increased sharply beyond 4 V (-4V for backward) and subsequently reached saturation before 5 V for both voltage direction.
• the current saturation is attributed to the higher density of states in the conduction band than that of the excited electron.
• Phototransistors are built on photo-related free carriers under illumination. Hence, two working mechanisms may occur in this phototransistor device. One is photovoltaic effect which depends on the light absorption, exciton dissociation, electron and hole diffusion and electrode collections. 9 The other possible mechanism is photoconductive effect which results in a huge increse of carrer density in the nanowire ( ⁇ - ⁇ , where ⁇ 7 is the conductivity, ⁇ is the carrier mobility of the material, n is the carrier density and q is the charge of electron). Refer to the present system, no photovoltage was observed in the device despite a large photocurrent was formed. Hence the photoconductive effect is more significant and can be described by 14,29
• the drain current was controlled by the optical power density and the curves displayed good transistor behavior, consisting of a rapidly increased linear regime and a fully saturated regime, which is similar to the output characteristics of a traditional field effect transistor modified by gate voltage. Except here the light intensity plays the role of voltage gating.
• the results suggest that the incident light could be employed to replace the gate voltage, V DS , as an additional terminal to control the output level of the transistor, indicating an effective approach to achieve current modification and signal magnification in a single nanowire device for future low-cost, nano-scale photoelectric integration.
• the responsivity, R res an important parameter of phototransistors, could be calculated by 30
• the device also showed a good phototransistor behavior.
• the output current was well controlled by incident light power and saturation is achieved at higher voltage regime. Again, the unidirectional property of the existing current in K x Mo0 3 nanowire resulted in the slightly higher output current compared with the negative voltage region. Careful analysis of the output characteristics could indicate more details about the phototransistor mechanism, as shown in FIG. 2f and h.
• the regions for - 4.5V ⁇ V DS ⁇ 0 and 0 ⁇ V DS ⁇ 4.5V were zoomed in and it is clear that the photo-related current is only available in the case of drain voltage beyond 4 V (or -4V).
• the existing output curve under dark field was further investigated.
• a small current was detected at around 4 V and increased with the increasing voltage thereafter, which indicates a typical Schottky contact characteristic curve. Therefore, the whole output characteristic is predicted to be the combination of two possible mechanisms: one is the unidirectional output caused by nonuniform intrinsic potential distribution (denoted as part I) and the other is a normal semiconductor output with Schottky contact (part II), as shown in FIG. 3. Since the magnitude of the output current related to part I is much higher than that of part II, part I dominated the output characteristic curve in dark condition by showing the unidirectional tendency of the entire output current.
• part I showed little fluctuation with incident light power, it changed dramatically with thermal variation.
• the output current was measured with temperature varying from 77 K to 330K and the corresponding l D s curves were recorded.
• the V D s values were kept at 2 V, 3 V and 4 V respectively at which region l DS is hardly affected by the incident light.
• An exponential behavior of the output current with increasing temperature is demonstrated for various applied bias. The electron movement almost came to a stop below 100K and increased rapidly with rising temperature from 200 K to 330 K.
• the corresponding values of ln(l ds ) display a quasi-linear relationship with the increasing temperature, as shown inset of FIG. 4a.
• the current-temperature dependence of the nanowire follows the thermal-activation model, 32,34
• FIG. 4a shows the typical characteristic curves under 3 mW/cm 2 illumination at different temperature (part II).
• the promising phototransistor performance is also supported by the ultra-fast photo- response of the KJv1o0 3 nanowire to light.
• the light source was turned on and off for 5 s respectively.
• FIG. 5a the device presented a perfect switching on/off behavior.
• Each photoresponse cycle consists of three distinct stages: a sharp rise, a steady state and a sharp decay process to original state with an on/off current ratio of about 10 4 under 4 mW/cm 2 illumination.
• the phototransistor is demonstrated to response to full spectrum regime.
• the photoresponse characteristics were investigated through employing a series of filters with wavelengths of 488 nm, 500 nm, 514 nm, 532 nm, 550 nm, 570 nm, 600 nm, 633 nm and lasers with wavelengths of 325 nm, 532 nm, 808 nm and 1064 nm. As can be seen in FIG.
• the phototransistor is able to respond to all light wavelengths from 488-1 100nm using a light filter and 532, IR (808, 1064) laser for verification.
• the representative results and photocurrent as a function of wavelength are shown in FIGS. 17 and 18.
• the photoresponse of the device does not show obvious fluctuation in the visible spectrum regime.
• the K x Mo0 3 nanowire device is much more sensitive for the visible spectrum regime which promotes this kind of devices toward a myriad of applications in daily life.
• the ⁇ * ⁇ 0 3 phototransistor shows consistent device performance.
• K-intercalated Mo0 3 nanowire has been successfully synthesized through a simple but effective one-step vapor deposition method.
• EDX analysis confirmed the decreasing trend of K concentration along the nanowire growth direction which enables the unidirectional, diode-like behavior of the dark current. This part of current is found to be irresponsive to the change of light intensity but to the temperature.
• the extreme shallow donor created by the intercalation of K enables an ultra-sensitive and sub-millisecond photocurrent response.
• the nanowire may also be synthesized at a furnace temperature ranging from 300 to 900 °C at a dwelling time ranging from 10 minutes to 12 hrs.
• other alternatives may be used as the substrate for the molybdenum oxide phototransistor, such polyethylene terephthalate (PET), and other low-cost plastic and bendable substrates. Similar results for a device made on PET are shown depicted in FIG. 13. REFERENCES

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