Thermal doping by vacancy formation in nanocrystals

Abstract

The invention generally relates to methods of thermal doping by vacancy formation in nanocrystals, devices and uses thereof.

Claims

1. A method for vacancy doping of a nanoparticle material, the method comprising: treating a nanoparticle material, under oxygen-free conditions, and at a temperature below 380K, said temperature being selected to permit formation of vacancies within the nanoparticle material, while avoiding fusion of said nanoparticles.

2. The method according to claim 1, the method being performed such that at least one electrical property of a nanoparticle material is amplified or attenuated.

3. The method according to claim 2, wherein the electrical property is selected from free charge carriers, conductance, impedance, resistance, voltage, current, potential and polarization.

4. The method according to claim 1, wherein vacancy doping amplifies conductance of the nanoparticle material.

5. The method according to claim 1, wherein the number of vacancies per nanoparticle is between 1 to 1,000 vacancies per nanoparticle.

6. The method according to claim 1, wherein the number of vacancies ranges from 0.001% to 20%, 0.01% to 20%, 0.1% to 20% or 1% to 20% of the number of lattice sites in the nanoparticle.

7. The method according to claim 1, wherein vacancies are achievable by laser beam radiation, the laser beam being selected to have a beam wavelength corresponding to the wavelength range of the absorption spectra of the nanoparticle material.

8. The method according to claim 1, further comprising a step of forming an array or a pattern of nanoparticles prior to or after vacancy doping.

9. The method according to claim 8, comprising: obtaining the nanoparticles array; and thermally treating said nanoparticles array so as to cause vacancy doping.

10. The method according to claim 9, wherein the thermal treatment is achieved by laser induced heating.

11. The method according to claim 10, wherein the method forms a pattern of vacancy doped nanoparticles in the nanoparticles array.

12. The method according to claim 11, wherein the pattern is conductive.

13. The method according to claim 11, wherein the pattern is formed on an electronic device.

14. The method according to claim 1, further comprising a step of doping a vacancy doped nanoparticle with at least one foreign atom.

15. The method according to claim 14, wherein the foreign atom is Li or Mg or Na or K or Rb or Cs or Be or Ca or Sr or Ba or Sc or Ti or V or Cr or Fe or Ni or Cu or Zn or Y or La or Zr or Nb or Tc or Ru or Mo or Rh or W or Au or Pt or Pd or Ag or Co or Cd or Hf or Ta or Re or Os or Ir or Hg or B or Al or Ga or In or Tl or C or Si or Ge or Sn or Pb or any combination thereof.

16. The method according to claim 1, wherein the nanoparticle is a colloidal nanoparticle of at least one material selected from metal, insulator, and a semiconductor material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

(2) FIGS. 1A-B:

(3) FIG. 1A is powder XRD scattering curves of as synthesized Cu.sub.2S NC films under inert environment compared to standard diffraction spectra of stoichiometric bulk Cu.sub.2S (chalcocite) and the copper depleted Cu.sub.1.96S (djurleite) phases, lower and upper axis, respectively. The peak at 2=40.8 is unique for the low chalcocite Cu.sub.2S phase (marked with arrows).

(4) FIG. 1B is the absorbance spectra of highly stoichiometric as-synthesized Cu.sub.2S NCs (curve a) which do not exhibit a near IR plasmon feature signifying low charge carrier concentration. After exposure to oxygen (curve b) the appearance of a local surface plasmon is seen, signifying a significant induction of copper vacancies.

(5) FIGS. 2A-F: Cu.sub.2S NC array thermal doping effect.

(6) FIG. 2A High resolution scanning electron micrograph (HRSEM) of NCs array between electrodes (arrows) (see inset in FIG. 2D); scale bar 200 nm.

(7) FIG. 2B HRSEM of Self assembled highly ordered superlattices a few microns in size; scale bar 150 nm (left hand side; marked b) and 5 micron (right hand side; marked e). SAXS raw (FIG. 2B left hand side inset) and

(8) FIG. 2C is the azimuthally integrated data measured on NC arrays showing peaks manifesting superlattice ordering. The first peak is composed of two peaks one at q0.49 nm.sup.1 which is attributed the hexagonal ordering of the Cu.sub.2S NC monolayer parallel to the substrate. The second peak at q0.53 nm.sup.1 is attributed cubical ordering of the Cu.sub.2S NC monolayer perpendicular to the substrate. The oriented 2D pattern presents features of hexagonal order, which is confirmed by the relative positions of the harmonics. Scattering curves prior (curve a) and after (curve b) thermal doping are compared, indicating a slight decrease, by 0.14 nm, in the inter-NC distance.

(9) FIG. 2D depicts film conductance at 1V vs. temperature, exhibiting minor temperature dependence below 300K (solid curve). At elevated temperatures (350-400K), film conductance increases significantly. Error bars depict the conductance changes at these temperatures with annealing time (FIG. 2F).

(10) FIG. 2E presents plots of I.sub.d versus V.sub.sd at 20K (curve a), and after the thermal doping process at 380K (curve b), showing increase in conductance by 6 orders of magnitude. Both data sets (FIG. 2D-E) were acquired on the disordered arrays. Cu.sub.2S NCs were 14 nm in diameter with hexagonal truncated bi-prism geometry, as depicted in the HRTEM image and cartoon presented in (FIG. 2D inset).

(11) FIG. 2F presents I.sub.d measurements vs. treatment time, during the thermal doping process measured at 380K. The increase in current exhibit a logarithmic or stretched exponential time dependence (left frame). After 20 minutes of thermal treatment, a significant contribution to the conductance is already measured.

(12) FIGS. 3A-B: Temperature dependence of conductance in the thermal doping process.

(13) FIG. 3A G(T) curves after the thermal treatment at various measurement temperatures. Repeated cooling (triangles) and heating (circles) indicate no hysteresis effect below 350K, demonstrating the irreversibility of the conductance increase.

(14) FIG. 3B Arrhenius-like plot of the natural logarithms of the conductance vs. the inverse of the thermal doping temperature, for three different samples measured at room temperature (squares, circles, triangles) and for one sample at various measurement temperatures (solid curve300K, dashed curve200K, dotted curve100K). The slope of the fitted lines according to Arrhenius equation yields E.sub.a/k.sub.B where k.sub.B is the Boltzmann constant and E.sub.a is the activation energy for forming a Cu vacancy. The slope of the fitted lines yields the Cu vacancy formation energy E.sub.vf1.6 eV.

(15) FIGS. 4A-B:

(16) FIG. 4A shows Thermo-gravimetric analysis (TGA) curve (solid curve) and derivative (dotted curve), showing no detectable reduction in weight upon heating up to temperatures of 500K. Significant weight losses are recorded only above 530K and around 850K these are attributed to organic ligands loss.

(17) FIG. 4B Differential scanning calorimetry (DSC) curve showing a reversible peak around 360K in the heating cycle (curve a) and 340K in the cooling cycle (curve b). These are attributed to the documented Cu.sub.2S low-to-high chalcocite phase transition, but not the ligands loss which commences at higher temperatures and is not reversible.

(18) FIG. 5: High resolution transmission electron micrograph (HRTEM) depicting Cu.sub.2S NC array after the thermal doping process showing the NCs are intact with a crystalline structure. No evidence for sintering or fusing that may contribute to the inter particle charge transfer mechanism is observed. Scale bar is 10 nm.

(19) FIGS. 6A-C:

(20) FIG. 6A AFM (atomic force microscopy) topography;

(21) FIG. 6B optical phase micrographs; and

(22) FIG. 6C HRSEM (high resolution scanning electron microscopy) micrographs depicting Cu.sub.2S NC deposited on flame annealed atomically flat Au substrates. These self assembled highly ordered monolayers were used for the CAFM (conductance AFM) local conductance measurements. Scale bars are 70/70/50 nm respectively.

(23) FIGS. 7A-F: Single NC thermal doping effect.

(24) FIG. 7A, B: C-AFM measurements (inset scheme in FIG. 7B) performed on an ordered monolayer of Cu.sub.2S NCs deposited on annealed Au substrate. AFM topographic (left) and current (right) micrographs before thermal doping, measured with 3 V tip-sample bias voltage, scale bar 50 nm Cross-section along the line drawn in the current image is plotted before (FIGS. 7A, B curve a), and after thermal doping at 380 K (FIG. 7B curve b), showing 2 orders of magnitude local enhancement.

(25) FIG. 7C Histograms of current values from CAFM local current measurements measured on hundreds of NCs. Shown are the results for as synthesized NC (curve a), after thermal treatment at 350K (curve b) and at 380K (curve c). The data depicts a clear increase in conductance for the 380K treated sample. The steep increase around 20 nA originates from saturation of the CAFM current amplifier around these values, suggesting even higher conductance values for thermal doped NCs.

(26) FIG. 7D inset STM (scanning tunneling microscopy) current map of a single Cu.sub.2S NC, scale bar 4 nm FIG. 7D dI/dV-V tunneling spectra, measured at 4.2 K before (curve a) and after (curve b) thermal doping.

(27) FIG. 7E Histograms of relative band-offset values measured on 24 single NCs, comparing the thermally doped and pre-doped populations. An increase in the relative band offset toward positive values is observed after thermal doping akin to p-type doping.

(28) FIG. 7F (left, middle, right) Additional representative STS (scanning tunneling spectroscopy) curves measured on single Cu.sub.2S NCs before (curves a) and after (curves b) the full scale thermal doping process, (heating to 380K). The data depicts the spread in the relative band offset shifts and the appearance of in-gap states. The distribution of band offset shifts is presented by the histogram in FIG. 7E.

(29) FIGS. 8A-C:

(30) FIG. 8A exhibits local thermal doping conducted using focused laser light as the heat source and under inert environment, as depicted in the scheme.

(31) FIG. 8B presents a Current(I.sub.d) measurements vs V.sub.sd resulted in currents of 25 pA at 10 V when measured at 300K before the thermal doping process.

(32) FIG. 8C presents a Current(I.sub.d) measurements vs V.sub.sd After illuminating the array with an intense focused laser beam, a pronounced conductance increase was observed, yielding currents of 1 A at 0.1 V when measured at 300K. The increase in conductance was irreversible when cooling down the sample (200K, 20K).

(33) FIGS. 9A-C: Optical local thermal doping.

(34) FIG. 9A Normalized film conductance vs. temperature before (native) and after irradiation by laser powers of 1, 5, and 7 mW. G was normalized by the respective G.sub.0 at 300K of the as-prepared film. Irradiation by 7 mW increases the film conductance by six orders of magnitude. (FIG. 9A inset) Optical micrograph showing super-lattices deposited on micro-electrodes; circle marks the local laser irradiated thermally doped region, scale bar is 8 m.

(35) FIG. 9B top KPM (Kelvin Probe Microscopy) potential and FIG. 9B bottom topography maps of laser thermally doped NC array. Scan area is 6 m.sup.2. FIG. 9B middle Line cross-sections taken along the lines marked in b and d. The potential cross-section (curve a) depicts a 80 mV increase with respect to the background, whereas the topography cross-section (curve b) shows that no change was inflicted to the array morphology due to laser irradiation.

(36) FIG. 9C KPM potential map depicting pattered doping achieved via optical thermal doping and controlled movement of the substrate stage, scale bar is 5 m.

DETAILED DESCRIPTION OF EMBODIMENTS

(37) Tuning electrical properties of semiconductor nanocrystals (NCs) is of importance both from fundamental and practical points of view. Various doping methods for NCs have been demonstrated by introducing dopants into NCs or by adding charge carriers through surface modifications. Lattice vacancies may also lead to excess charge carriers enabling a dopant-less method for tuning the electrical properties of NCs.

(38) The novel approach of the invention to doping of NC films is termed thermal doping. This approach is demonstrated for Cu.sub.2S NC arrays where moderate temperature treatment leads to significant conductance enhancement in a controlled manner.

(39) Copper sulfide is an interesting semiconductor material widely studied for its structural complexity that presents itself in a rich phase diagram with many structural phase transitions. For example, above 105 C. stochiometric Cu.sub.2S is high chalcocite ( phase), in which the sulphur atoms are arranged in a hexagonal lattice while the copper atoms are virtually fluid. Below 105 C., the hexagonal sulfur lattice stays rigid while the copper atoms pack in a complex interstitial manner, giving rise to a lower symmetry monoclinic phase ( phase), also known as low chalcocite. In addition, Cu.sub.2S has a tendency for Cu deficiency which originates from the low chemical potential of Cu(0). Vacancies can form by the loss of copper atoms. The results in a series of Cu depleted materials, for example, the near stochiometric Djurleite phase (Cu.sub.1.96S), which is crystallographically distinct from low chalcocite and has a monoclinic structure with 248 copper and 128 sulfur atoms in the unit cell. Due to high ionic mobility of Cu ions, vacancies can cluster together in groups of four per Cu.sub.20S.sub.12 unit, with the remainder of the copper ordered similarly to low chalcocite.

(40) Thermal doping was demonstrated for Cu.sub.2S NC films via temperature induced copper vacancy formation at moderate temperatures. The doping effect is proven using transport measurements, conductive atomic force microscopy (CAFM) and scanning tunneling microscopy (STM).

(41) p-type doping was identified by scanning tunneling spectroscopy (STS) of single Cu.sub.2S NCs and is attributed to formation of Cu lattice vacancies within the NCs. Unlike previous methods for aliovalent doping, which introduce either substitutional or interstitial impurities, in the thermal doping approach no additional impurities are introduced to the NC; rather the opposite process takes place, the expulsion of atoms from the intrinsic material. Furthermore, laser induced heating was used to conduct the thermal doping process, opening the path to patterning via use of local heating by a focused laser beam in the far or near-field.

(42) Highly monodisperse and faceted 14 nm Cu.sub.2S NC were synthesized by a high temperature reaction using Cu(acac).sub.2 (copper acetylacetonate) as Cu precursor, and dodecanthiol serving both as sulfur precursor and surfactant (details in the methods section below, absorption and X-ray diffraction are shown in FIGS. 1A-B, respectively).

(43) The NCs were deposited from solution using a controlled slow solvent evaporation method on Si/SiO.sub.2 substrates with pre-patterned electrodes (Cr/Au, 2 m spacing) forming 150 nm thick films (FIG. 2A). Tuning the NCs organic ligands layer concentration enables us, through self-assembly, to achieve also highly ordered super-lattices as depicted in FIG. 2B, and verified by distinct peaks in the small-angle X-ray scattering (SAXS) data (FIG. 2C). NCs were kept under inert environment, as exposure to oxygen may also induce copper vacancies in a competing process of oxidation where a plasmon band was observed due to free carriers.

(44) Drain-current (I.sub.d) measurements were conducted under high vacuum in a variable temperature probe-station. Source-drain bias voltages (V.sub.sd) of 1-10 V were used, yielding initially very low conductance values of 1-10 pS at room temperature (RT). Then, to induce irreversible doping via controlled introduction of vacancies, through the thermal doping process, the film was heated in controlled manner to moderate temperatures. The film was heated and conductance (G) measurements between 340 and 400K were preformed (FIG. 2D). Remarkably, films treated at 380-400K showed 4-6 orders of magnitude increase in G. FIG. 2E depicts I-V curves demonstrating current enhancement at 10 V from 6.4 pA at 20K (curve a) to 30 A at 380K (curve b).

(45) The results clearly indicate that the source for this remarkable increase of film conductance at such moderate temperatures originates from the unique thermal doping approach. The rise in film conductance is irreversible; cooling the film does not reduce G back to its original values, as depicted in FIG. 3A showing G(T) curves measured for the NC films after treatment at increasing temperatures between 350 and 390K, exhibiting exponential increase with temperature (used for the thermal doping process; thermal process temperatures are marked with arrows). FIG. 3B portrays the logarithm of conductance vs. the inverse of the treatment temperature for three different samples measured at RT and also for one sample at various measurement temperatures. All data sets exhibit Arrhenius-like dependence for the thermal process, with a common slope corresponding to an energy of 1.6 eV.

(46) High temperature annealing was previously applied to NC films for increasing their conductance, by removing capping organic ligands, thus reducing the inter-NC spacing. This annealing, however, was achieved at significantly higher temperatures, 500K, typically leading to removal of capping organic ligands and reduction of inter-particle spacing by 0.5 nm, and in some cases even to sintering.

(47) Here, the irreversible remarkable increase in conductance occurs at moderate treatment temperatures and may originate both from an inter-NC effect, of reducing the NC separation upon heating, and an intra-NC effect leading to enhanced number of charge carriers, namely NC doping. The former effect was estimated, showing that this effect is the minor one. To directly assess the change in inter-NC spacing, SAXS was used. FIG. 2C depicts scattering intensity versus the magnitude of the reciprocal lattice vector, q, before and after the thermal treatment. SAXS measurements indeed indicate slightly reduced distances between adjacent NC, where distance of 14.78/14.64 nm between centers of adjacent NC before/after heating, were measured. The SAXS data indicate only a slight reduction by 0.14 nm in inter-NC spacing upon thermal treatment at 380K. Therefore, unlike the prior work, here the increased conductance after heating cannot be explained solely nor mainly by the decrease in inter-particle separation. This conclusion is supported by an estimate of the tunneling probability of charge carriers between adjacent NCs. When the tunneling barrier width is decreased by d, the inter-NC tunneling probability increases by e.sup.2.Math..Math.d, where in our case .sup.1 is 0.97 , similar to the tunneling decay value through saturated alkane chains (methods section below). For the measured d=0.14 nm, the tunneling probability increases only by a factor of 20. Moreover, the use of relatively low temperatures does not lead to chemical changes of the ligands layer (FIG. 4). The decrease in inter-particle separation is therefore assigned to residual solvent removal accompanied by enhanced packing and reordering of the interpenetrating alkane chains on the NC surfaces. The six orders of magnitude increase in NC film conductance may also be associated to physically fusing (sintering) individual NCs into a two dimensional material [14]. In the present case rather low temperatures were used, which are insufficient for sintering. Recently, Evers et al. reported directed attachment and fusing of PbSe NCs into two dimensional materials during self assembly [15]. In this case, HRTEM and SAXS measurements of the film prior and post to the thermal doping process showed no evidence for fusing between neighboring NCs, which in the present case are coated by strongly bound dodecanethiol ligands thus NCs sintering is unlikely (FIG. 5). Therefore, the inter-NC changes upon thermal treatment can account only for a small fraction of the observed 4-6 orders of magnitude conductance enhancement.

(48) The thermal treatment that is proposed herein is preformed at much milder temperature than the prior art for which the ligand layer is generally not removed (see SAXS). The outcomes of this process are in general of similar functionality as to the present process. The two methods could be differentiated by looking into effects that require the ligand layer intact. For example, quantum confinement effects may be diminished if ligands are removed and sintering of NCs occur. In addition the removal of the ligand layer may reactivate surface defects that were passivated by the ligands, a process that may increase the recombination rate of excitons and further degrade performance of electrical devices.

(49) Instead of an inter-particle effect, the increased conductance after the thermal doping treatment is an intra-particle effect of doping by the creation of Cu vacancies at the moderate temperatures of the thermal doping process. This is related to the Cu.sub.2S thermodynamic tendency to form Cu vacancies leading to p-type doping. Charge carriers concentration in the Cu.sub.2S is associated to the amount of Cu vacancies, where increased number of vacancies can improve the conductance of the NC film at these moderate temperatures.

(50) First, careful control of the thermal doping temperature enables us to tune the film conductance values as depicted in FIG. 3A. Moreover, FIG. 3B depicts the logarithm of conductance vs the inverse of the thermal doping temperature. This is depicted for three different samples (square, circle, triangle) and also at various temperatures (as indicated).

(51) A Quantifying Parameter for Distinguishing Thermal Doping from Other Effects.

(52) The formation of vacancies in crystals via thermal treatment has a well defined guiding formula.
nN.Math.e.sup..sup.vf.sup.lk.sup.B.sup.T.sup.ld,

(53) where n is the number of vacant atoms sites and N is the total sites in the crystal. The formation energy .sub.vf is a material property and k.sub.B is the Boltzmann constant.

(54) In materials for which the vacancies are directly associated to the charge carrier concentration, one can monitor the vacancy formation through the increase in conductance and verify the above law. Formation energy of 1-2 eV is reasonable while much smaller formation energies are probably associated with activation of inner or other conductance levels.

(55) The linear relation follows Arrhenius activation dependence for the introduced thermal doping process that allows extracting the activation energy for Cu vacancies formation to be 1.6 eV per vacancy.

(56) Next the contribution of intra-particle effects to the remarkable increase in conductance upon thermal treatment of the NC films was considered. To this end, a study of the electronic properties at the single particle level was conducted by local probe measurements using both conductance atomic force microscopy (C-AFM) and scanning tunneling microscopy (STM). Local conductance of single NCs were measured via C-AFM while applying 3 V bias between the conducting tip and a gold surface on which NCs were deposited, yielding self-assembled NC monolayers (FIGS. 6,7). FIG. 7A exhibits a topography and current map of a typical array. Cross-sectional analysis of the current map reveals approximately two orders of magnitude increase in the single NC conductance before and after the thermal treatment (FIG. 7B). Statistical analyses for hundreds of thermally treated single NCs corroborates these results (FIG. 7C). The increase of conductance at the single NC level further shows that the thermal doping process induces Cu vacancies, and modifies the NCs inherent electronic properties, realizing a doping effect, as in the Cu.sub.2S NC array. The significant increase of conductance at the single NC level shows that the thermal treatment process induces an intra-particle modification leading to the introduction of charge carriers.

(57) For further understanding of these intra-particle changes, STM and STS were used. Dilute samples of Cu.sub.2S NCs were deposited on atomically flat Au surfaces, where isolated NC can be measured. The STM tip was positioned above a single Cu.sub.2S NC realizing a double barrier tunnel junction configuration, and I-V spectra were acquired. FIG. 7D portrays typical dI/dV-V tunneling spectra, measured at 4.2K before (curve a) and after (curve b) thermal treatment. The dI/dV-V spectra are proportional to the local density of states. The spectrum for pre-thermal treated NCs depicts typical semiconductor density of states with a clear band-gap. After the thermal treatment, a finite density of states inside the band gap is identified. Most notably, a distinct offset of the band-edges toward higher energies is observed. This offset is demonstrated by statistical analysis of the band-edge shifts (FIG. 7E; FIG. 7F shows additional STS curves), where in the thermal treated NCs (grey bars), a statistically significant increase of the band offsets towards positive values is evident, compared with the pre-treated NCs (black bars). This analysis indicates a shift of the Fermi energy towards the valence band, akin to p-doped semiconductors. This result further corroborates the formation of vacancies via thermal doping, which are associated to the hole conduction in the NC. These aforementioned AFM and STM results corroborate the intra-particle electronic change due to the thermal doping treatment leading to p-type doping in this case.

(58) p-type behavior of Cu.sub.2S is known in the bulk and attributed to Cu vacancies, which form due to the low chemical potential of Cu(0). This behavior results in a series of Cu depleted substances with a rich phase diagram, for example, the near stochiometric Djurleite phase (Cu.sub.1.96S), resulting in free hole charge carriers. In bulk Cu.sub.2S thin films, thermal annealing at 430K increased the film conductance only by a factor of two. The vacancy formation is therefore significantly enhanced in Cu.sub.2S NCs due to facile diffusion of Cu atoms to the surface. Given the significant change in conductance and the related p-type behavior induced after the thermal treatment, the process was coined thermal doping.

(59) A simplified model was conjectured for the hole conductance G in the Cu.sub.2S NCs array and its dependence on the thermal doping temperature T.sub.td,
G(T.sub.td)e.sup..sup.vf.sup.lk.sup.B.sup.T.sup.td.Math.e.sup.2d(T.sup.td.sup.),(1)

(60) where the first exponential expression represents the intra-particle term of copper vacancies formation with a formation energy .sub.vf(k.sub.B is the Boltzmann constant). The second term is the aforementioned inter-particle probability for electron tunneling between neighboring NCs, which contributes to increased conductance upon the thermal treatment by only a factor of 20, and therefore can be neglected leading to the approximate dependence

(61) $\begin{array}{cc}\frac{d\ln G}{d\left(1/T_{\mathrm{td}}\right)}-\frac{{\mathrm{.Math.}}_{\mathrm{vf}}}{k_B}.& \left(2\right)\end{array}$

(62) This relation is indeed fully consistent with the observed Arrhenius-like activation in FIG. 3B, allowing to derive the Cu vacancy formation energy, .sub.vf1.6 eV. Using this value, an increase by a factor of 510.sup.4 was estimated in the hole density following the thermal doping process at 380K, compared with the as-prepared NC array. This increase translates to a 280 meV shift of the Fermi energy towards the valence band, similar to the values observed in the STS measurements on single NCs (see the methods section below).

(63) Cu.sub.2S NCs were synthesized in a method that yielded minimal or no copper vacancy content and were highly stoichiometric, as supported by the lack of optical surface plasmon (FIG. 1B) and by variable temperature powder XRD measurements (FIG. 1A). The XRD data is consistent with Cu depletion from the initial stoichiometric Cu.sub.2S low chalcocite phase, towards a Cu depleted phase. In a conservative estimate of a single vacancy (hole) per 1000 as synthesized NCs, and assuming a free electron model with linear relation between conductance and vacancy (charge carrier) concentration, 50 vacancies (holes) were estimated per a 14 nm thermally doped NC.

(64) Thermal doping of semiconductor NC films has diverse applications in bottom-up solution based preparation of electronic devices. Moreover, the ability to locally dope specific regions of the film while maintaining different conductance nearby, may serve as a patterning method for profile doping. Local Cu vacancy formation was implemented via thermal doping by using a focused laser as the heat source (FIGS. 8, 9A). At lower laser powers (1-5 mW) conductance increased by an order of magnitude. Remarkably, after exposure to a laser power of 7 mW, the film conductance increased by 6 orders of magnitude, similar to the thermal doping process induced by heating which was discussed above. This demonstrates that local thermal doping is readily possible via laser irradiation. After exposure to laser intensity of 2.Math.10.sup.5 W/cm.sup.2, the film conductance irreversibly increased by 6 orders of magnitude, similar to the thermal doping process discussed above.

(65) To further characterize the laser induced thermal doping process, NC arrays were assembled on and indium-tin-oxide (ITO) film. AFM was equipped with inline optics which enabled correlation of the AFM scan with the optical axis used for laser illumination. The samples were thermally heated via laser illumination (10 mW, 532 nm) after which the illuminated area was characterized with the AFM. Both topography and Kelvin probe microscopy (KPM) data were measured using a dual pass technique (more details are provided in the methods section of the supporting information). FIG. 9B top, 9B bottom depict 3D maps of the potential and topography of the thermally doped area. The line cross-section taken along the potential map (FIG. 9B top) presented in FIG. 9B middle depicts a 80 mV change in the potential of the heated NC array (curve a), while no change can be detected in the corresponding topography scan (curve b). The increase of the surface potential in the illuminated region is consistent with p-type doping and the concomitant increase of hole concentration in the NCs at this region. KPM data thus indicate that also in the case of laser annealing the conductance enhancement is predominantly due to intra-particle p-doping. Finally using a controlled movement of the sample stage patterned doping was generated, and characterized with the KPM technique, as portrayed in FIG. 9C. This demonstrates the feasibility of local thermal doping via laser irradiation.

(66) The use of aperture and aperture-less near-field microscopy allows achieving high resolution doping profiles even down to 20-50 nm scale. For example, this enables specific patterning of a line of NCs within an array which is a highly challenging task.

(67) In addition, local doping of the NC film is demonstrated via the use of a focused laser beam, serving as the heating source. This approach may be used in order to fabricate NC based electronic and optoelectronic devices by using a laser beam to pattern locally the film's doping profile and hence, for example, conductance properties. Thermal doping of NC films leading to vacancy formation is of importance for the bottom-up fabrication and function of diverse electronic and optoelectronic devices such as transistors, solar cells and light emitting diodes in which electrical properties of deposited NC films are typically much less controllable than in the traditionally fabricated electronic devices. Furthermore, the exceptional low temperatures needed for the thermal doping leading to vacancy formation in Cu.sub.2S may further benefit printed plastic electronics on flexible substrates which are incompatible with higher process temperatures. Moreover, in additional embodiments, thermal doping is expanded by following the thermal doping process with a step of reacting the thermal doped NC with other materials to insert impurities into the NCs. For example with impurities that would lead to n-type behavior.

(68) Cu.sub.2S is a binary copper chalcogenide, and is a mother compound for a family of other semiconductors, for example CuInS.sub.2, CuCaF.sub.2, CuInSe.sub.2 and CuInGaSe.sub.2. These developments open the path to fabrication of diverse electronic, optoelectronic and solar cell devices using the thermal doping approach, which is also applicable for these materials.

(69) Methods

(70) Materials:

(71) Cu(acac).sub.2 (99.99%), dodecanethiol (98%), hexamethyldisilazane (99.9%), chloroform (Anhydrous 99%), and methanol (Anhydrous 99.8%) were purchased from Sigma Aldrich.

(72) Si wafers were purchased from Virginia semiconductor inc.

(73) Synthesis of Cu.sub.2S NCs:

(74) Cu.sub.2S NCs, were synthesized following the procedure described in Ref (1). Briefly, Cu.sub.2S seeds are formed by the decomposition of Cu(acac).sub.2 in excess dodecanethiol under inert atmosphere at 200 C. The dodecanethiol acts as the solvent, surfactant and particularly as the sulfur source. The crude seed solution is washed repeatedly with chloroform and the purified precipitate is kept in the glove box. The washing process depletes the amount of the dodecanthiol ligands on the NC surfaces enhancing the natural tendency of these NCs to self assemble into ordered superlattices.

(75) Device Fabrication:

(76) Au/Cr electrodes were patterned using standard optical lithography methods onto highly p-doped SiO.sub.2/Si substrates (thermal oxide 100 nm) and also on glass substrates for the optical thermal doping process. Cr (2 nm adhesion layer) and Au (100 nm) electrodes were then thermally evaporated. Multiple electrode separations were tested. Typical distances between electrodes of 1.8 m (L/W ratios of 10-100) gave the best results. Prior to the Cu.sub.2S NC deposition, the substrates were cleaned with piranha solution and treated with hexamethyldisilazane molecules to increase surface hydrophobicity. Tight size distribution of the NCs and careful control over the ligands coverage on the NCs surface were important parameters in the self assembly process. Excessive cleaning of the organic protective layer (typically four washing sequences with chloroform), resulted in colloidally unstable NCs with tendency to aggregate and precipitate, due to NC surface ligand depletion, although the solution could be redispersed by long sonication treatment. A more subtle cleaning method with isopropanol (IPA) led to a stable solution in which the NCs dispersion was stable, due to higher concentration of organic ligands.

(77) NCs deposition was performed at room temperature and under inert atmosphere, with the ability to tune the evaporation rate of the solvent (typically 10 micron/min) to achieve slow deposition of the NCs for better coverage and ordering. Substrates were placed in a vial vertically to its flat bottom. The vial was then filled with NC solution and the solvent was left to slowly evaporate. The resulting film was washed with IPA and characterized using HRTEM, XHR-SEM (Magellan) AFM and SAXS. NC deposition from the chloroform cleaned solution resulted in dense arrays with low order (FIG. 2A), while deposition of the NCs from the IPA cleaned solution resulted in highly ordered superlattices (FIG. 2A).

(78) Electrical Characterization:

(79) Electrical measurements on NCs films were conducted using a closed cycle cryogenic probe station (Advanced Research Systems inc.). The measurements were performed in high-vacuum conditions (10.sup.4 mbar) and in well controlled (sample) temperatures ranging between 20 and 400K. Source drain voltages of 1-10 V were applied by a voltage source (Keithly model 2400). Drain current was amplified by a current amplifier (DL Instruments 1211) and measured with a digital multimeter (Keithly model 2100). Sample temperature was controlled by a Lakeshore 340 controller.

(80) Conductive-AFM Measurements:

(81) The NCs solutions were drop cast from dilute solution onto a flame-annealed Au(111) substrate and let dry, resulting in a monolayer of Cu.sub.2S films. The samples were then promptly inserted into a (Solver P-47 NTMDT) AFM in N.sub.2 rich environment. Measurements were conducted in contact mode using TiPt coated Si tips (csc38/TiPt masch) with typical force constants of 0.03 N/m. High set-point values and slow tip-sample approach parameters were used in order to keep the Cu.sub.2S array intact under the contact mode measurement conditions. Scan rate of 1 Hz and tip bias of 3 V were used. Topography and current measurements were acquired simultaneously. The thermal doping procedure was conducted inside the AFM chamber under inert environment using a temperature controlled heated sample holder.

(82) STM and STS Measurements:

(83) For STM measurements, the NCs solutions were drop cast from an ultra dilute solution onto a flame annealed Au(111) substrate and let dry. The samples were promptly inserted into a homemade low temperature STM with RHK control electronics, where isolated single Cu.sub.2S NC could be measured. The STM measurements were performed at 4.2K, using PtIr tips, in He exchange gas. Tunneling I-V characteristics were acquired after positioning the STM tip above individual NCs, realizing a double barrier tunnel junction (DBTJ) configuration, and disabling momentarily the feedback loop. The dI/dV-V tunneling spectra, proportional to the local DOS, were numerically derived from the measured I-V curves. The topographic images were acquired with current and sample-bias set values of I0.1 nA and V1 V. Thermal doping procedure was conducted outside the STM under the same conditions as was performed for the conductance measurements. 9 and 14 different single Cu.sub.2S NCs were measured before/after the thermal doping process, respectively.

(84) Small Angle x-Ray Scattering (SAXS):

(85) NCs were deposited inside a 1.5 mm quartz capillary tube via the slow evaporation method discussed above. SAXS measurements were performed using an in-house setup described elsewhere. The pre-doped samples were measured and then promptly inserted into a temperature controlled probe station and thermally treated in the same manner the electronic samples were treated.

(86) Local Thermal Doping Process Via Laser Irradiation:

(87) Cu.sub.2S NC films were deposited on glass substrates with pre-patterned electrodes (see above). The samples were electronically characterized, placed in inert environment and illuminated with 532 nm laser which was focused using 20 objective (Nikon-CFI Plan Fluor 20, NA 0.5), to yield intensities of 2.Math.10.sup.5 W/cm.sup.2 for thermally doping the NCs. The sample stage was horizontally scanned using a JPK NanoWizard3 with TAO scanning stage during the optical irradiation to homogenize the heating effects along the illuminated gap. The sample was then inserted back to the probe-station for further electrical characterization.

(88) Wavefunction Decay Coefficient

(89) To estimate the conductance increase due to decrease in inter-NC distance between adjacent particles, we solve a simplified one dimensional square well potential tunneling problem. A barrier height, , of 5 eV was estimated for the alkyl chains, and a Cu.sub.2S hole effective mass, m.sub.h, of 0.8 m.sub.e, where m.sub.e is the free electron mass. , the wavefunction decay constant, was calculated as follows,

(90) $\begin{array}{cc}=\frac{\sqrt{2m_h}}{}=10.24{\mathrm{nm}}^{-1}& \left(1\right)\\ {}^{-1}=0.097\mathrm{nm}& \left(2\right)\end{array}$

(91) with this value of .sup.1 we next estimate the ratio of the hole tunneling probability,

(92) $\frac{}{{}_0},$
where .sub.0 and are the hole tunneling probabilities between adjacent NC before and after the thermal doping process, respectively

(93) $\begin{array}{cc}\frac{}{{}_0}=\exp \left(2.Math.{}^{*}d\right)=17.6& \left(3\right)\end{array}$

(94) Where d is the inter NC separation length change, extracted from the SAXS measurements (d=0.14 nm). Therefore, the inter-NC contribution is estimated to increase the conductance only by a factor of 20.

(95) Non Monotonic Temperature Dependence of the Conductance Between 20 and 300K

(96) A less pronounced, yet intriguing effect, is the non-monotonic temperature dependence of the conductance observed at temperatures between 20 and 300K, (FIG. 2A), exhibiting a slight local maximum of the film conductance at around 200K. Such behavior was observed for all Cu.sub.2S NC films studied, where the peak position varied between 175 to 210K. The G(T) curves are always reversible in this temperature range, with no hysteresis effect when repeated cooling and heating cycles are conducted, which differentiates it from the Cu vacancy thermal-doping process at elevated temperatures. The origin of this behavior is not completely clear, but may be associated with a change in the density of states observed in Cu.sub.2S thin films at around 175K. This change manifests itself by a change in the (activated) conductance dependence on temperature, becoming larger, namely, involving higher activation energy, above 175K. This corresponds to reduced density of states contributing to the thermal activated conductance in the films above 175K. This reduction in density of states is expected to decrease the density of available states for inter-particle tunneling or hopping in the NC films, resulting in a (local) reduction of the conductance at this temperature range.

(97) Calculation of the Fermi Energy Shift

(98) The extracted value of the Cu vacancy formation energy, .sub.vf1.6 eV, was used to estimate the increase in carrier density and hence the Fermi energy shift. The formation of vacancies in a crystal obeys the following formula:

(99) $\begin{array}{cc}nN.Math.\exp \left(-\frac{{\mathrm{.Math.}}_{\mathrm{vf}}}{k_B{T}_{\mathrm{td}}}\right),& \left(4\right)\end{array}$

(100) where n is the number of vacancies in the crystal, N is the total number of atoms in the crystal, .sub.vf is the energy required to remove an atom from a lattice site, k.sub.B is the Boltzmann constant and T.sub.td is the thermal doping temperature. A ratio of 5.Math.10.sup.4 increase was calculated in vacancies in the thermally treated NCs over the as synthesized NCs. In this calculation the contribution of matrix compression was taken into consideration, which contributes a factor of about 20 to the increase in conductance.

(101) The value of relative increase in charge carriers was used to estimate the Fermi energy shift using the expression:

(102) $\begin{array}{cc}{p}_v=n_i\exp \left(\frac{{}_i-}{k_{B}T}\right),& \left(5\right)\end{array}$

(103) where p.sub.v/n.sub.i is the ratio of concentration of holes after thermal doping compared with the as prepared NC array in the valance band (5.Math.10.sup.4), and .sub.i- is the energy shift in chemical potential between the doped and intrinsic case, getting a value of 280 meV. This shift is similar to values observed by us in the STS on single NCs.