Heavily doped semiconductor nanoparticles

Abstract

Herein, provided are heavily doped colloidal semiconductor nanocrystals and a process for introducing an impurity to semiconductor nanoparticles, providing control of band gap, Fermi energy and presence of charge carriers. The method is demonstrated using InAs colloidal nanocrystals, which are initially undoped, and are metal-doped (Cu, Ag, Au) by adding a metal salt solution.

Claims

1. A nanoparticle comprising a semiconductor material, the semiconductor material being doped with at least two atoms of a dopant material, wherein the at least two atoms of the dopant material are heterovalent to atoms of the semiconductor material, said at least two atoms of the dopant material being dispersed within the semiconductor material, and the nanoparticle is free of dopant islands within the nanoparticle and free of dopant islands on the surface of the nanoparticle.

2. A nanoparticle according to claim 1, wherein said at least two atoms of the dopant material alter the density of states of the semiconductor material.

3. The nanoparticle according to claim 1, wherein the nanoparticle consists of the semiconductor material doped with at least two atoms of the dopant material.

4. The nanoparticle according to claim 1, wherein an average length or diameter of the nanoparticle is in the range of 1 nm to 500 nm.

5. The nanoparticle according to claim 1, wherein the semiconductor material is selected from the group of elements consisting of Group I-VII, Group II-VI, Group III-V, Group IV-VI, Group III-VI, and Group IV semiconductors and combinations thereof.

6. The nanoparticle according to claim 5, wherein the semiconductor material is a Group I-VII material being selected from the group consisting of CuCl, CuBr, CuI, AgCl, AgBr, and AgI, or the semiconductor material is a Group II-VI material selected from the group consisting of CdSe, CdS, CdTe, ZnSe, ZnS, ZnTe, HgS, HgSe, HgTe, CdZnSe, ZnO and any combination thereof, or the semiconductor is a Group III-V material being selected from the group consisting of InAs, InP, InN, GaN, InSb, InAsP, InGaAs, GaAs, GaP, GaSb, AlP, AlN, AlAs, AlSb, CdSeTe, ZnCdSe and any combination thereof, or the semiconductor material is a Group IV-VI material being selected from the group consisting of PbSe, PbTe, PbS, PbSnTe, Tl.sub.2SnTe.sub.5 and any combination thereof, or the semiconductor material comprises an element of Group IV being selected from the group consisting of Si and Ge.

7. The nanoparticle according to claim 5, wherein the semiconductor material is selected from CdSe, CdS, CdTe, ZnSe, ZnS, ZnTe, HgS, HgSe, HgTe, CdZnSe, InAs, InP, InN, GaN, InSb, InAsP, InGaAs, GaAs, GaP, GaSb, AlP, AlN, AlAs, AlSb, CdSeTe, ZnCdSe, PbSe, PbTe, PbS, PbSnTe, Tl.sub.2SnTe.sub.5, RuS.sub.2, RuO.sub.2, MoS.sub.2, MoO.sub.3, RhS.sub.2, RuO.sub.4, WS.sub.2, WO.sub.2, Cu.sub.2S, Cu.sub.2Se, Cu.sub.2Te, CuInS.sub.2, CuInSe.sub.2, CuInTe.sub.2 and any combination thereof.

8. The nanoparticle according to claim 7, wherein the semiconductor material is selected from InAs, InP, InN, GaN, InSb, InAsP, InGaAs, GaAs, GaP, GaSb, AlP, AlN, AlAs, AlSb, CdSeTe, ZnCdSe and any combination thereof.

9. The nanoparticle according to claim 8, wherein the semiconductor material is selected from InAs, GaAs, GaP, GaSb, InP, InSb, AlAs, AlP, AlSb and InGaAs.

10. The nanoparticle according to claim 9, wherein the semiconductor material is InAs.

11. The nanoparticle according to claim 1, wherein the dopant material is selected from the group consisting of materials which atoms differ from atoms composing the semiconductor material by one or more valance electron(s).

12. The nanoparticle according to claim 11, wherein the dopant material is selected amongst metals and non-metal materials, said dopant being 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 P or As or Sb or Bi or O or S or Se or Te or Po or F or Cl or Br or I or At, or any combination thereof.

13. The nanoparticle according to claim 12, wherein the dopant is selected from Ag and Cu.

14. The nanoparticle according to claim 1, wherein the nanoparticle material is InAs and said dopant is selected from Ag and Cu.

15. The nanoparticle according to claim 1, wherein the number of dopant atoms dispersed in the nanoparticle ranges from 110.sup.18 atoms per cm.sup.3 to 110.sup.23 atoms per cm.sup.3.

16. The nanoparticle according to claim 1, wherein the number of dopant atoms per nanoparticle is between 2 to 500.

17. The nanoparticle according to claim 1, wherein the nanoparticle is a n-doped material having negative charge carriers, or a p-doped material having positive charge carriers.

18. A device comprising a nanoparticle according to claim 1, wherein the nanoparticle is incorporated into a layer and/or a region of the device.

19. The device according to claim 18, wherein the device is a bipolar transistor in a form selected from n-p-n, p-n-p and n-i-p type transistor.

20. The device according to claim 18, wherein the device is selected from a diode; a transistor; an electronic circuit component; an integrated circuit; a detector; a switch; an amplifier; a transducer; a laser; a tag; a biological tag; a photoconductor; a photodiode; a photovoltaic cell; a light emitting diode (LED); a light sensor; a display; and a large area display array.

21. A method for manufacturing a doped semiconductor nanoparticle, said method comprising providing an undoped nanoparticle comprising a semiconductor material, and contacting said undoped nanoparticle with at least one doping material under conditions permitting dispersion of at least two atoms of said doping material within said semiconductor material to form a doped semiconductor nanoparticle that is doped with at least two atoms of a dopant material, wherein said at least two atoms of the dopant material are heterovalent to atoms of said semiconductor material, and the doped semiconductor nanoparticle is free of dopant islands within the doped semiconductor nanoparticle and free of dopant islands on the surface of the doped semiconductor nanoparticle.

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) FIG. 1 presents diagrams describing the effects of heavy doping in bulk and nanocrystal semiconductors. FIG. 1A shows a scheme of the different influences of doping a bulk semiconductor for n-type (left) and p-type (right) dopants. In the figure: ABS is absorption onset; PL is photoluminescence onset; TS is tail states; Ef is Fermi energy; E.sub.g is modified band gap; and E.sub.g is unperturbed band gap. The shading shows state filling up to the Fermi energy. FIG. 1B is a sketch for n-doped nanocrystal QD with confined energy levels. Light and dark lines correspond to the QD and impurity levels, respectively. Left panel: the level diagram for a single impurity effective mass model, where E.sub.g is the quasi-particle gap in the doped QD, 1Se and 1Pe are the QD electron levels, and E.sub.d.sup.1S and E.sub.d.sup.1P are the impurity levels shifted below the corresponding QD levels by a shift D. Right panel: impurity levels develop into impurity bands as the number of impurities increases. Upper panel: sketch of the different impurity models.

(3) FIG. 2 shows the optical properties of doped InAs NCs. FIG. 2A is a TEM (transmission electron microscope) image of Ag-doped 3.3-nm InAs NCs. FIG. 2B shows normalized absorption spectra of three 3.3-nm InAs NC samples. Two of the samples had Cu (dotted line) and Ag (dashed line) solutions added to them resulting in metal/NC solution ratios of 540 and 264, respectively. These amounts correspond to 73 Cu atoms and 9 Ag atoms per NC. The third sample had a control solution (without metal salt) added to it (solid line). The inset shows the normalized emission spectra of these samples. FIG. 2C shows The energetic shift of the first exciton peak (solid symbols) and the emission energy (open symbols) against the number of impurity atoms per QD for InAs NCs with radii of 1.3 nm (left), 1.8 nm (center), and 2.1 nm (right). Black symbols correspond to Ag doping, gray symbols to Cu.

(4) FIG. 3 provides sample spectra for Cu doping. The normalized absorption (FIGS. 2A-C) and emission spectra (FIGS. 3D-F) for 3.3 nm diameter InAs NCs to which increasing amounts of Cu solution were added. The Cu:NC solution ratios are as follows: panel (FIGS. 3A and 3D) 0, 85, 170, 260, 340; panel (FIGS. 3B and 3E) are 0, 425, 510, 600, 680; panel (FIGS. 3C and 3F) are 0, 770, 850.

(5) FIG. 4 provides sample spectra for Ag doping. The normalized absorption (FIGS. 4A and 4B) and emission spectra (FIGS. 4C and 4D) for 3.3 nm diameter InAs NCs to which increasing amounts of Ag solution were added. The Ag:NC (nanocrystal) solution ratios in are as follows: panel (FIGS. 4A and 4C) 0, 50, 90, 170, 260; panel (FIGS. 4B and 4D) are 0, 332, 440, 850, 1000.

(6) FIG. 5 is the number of Cu atoms per NC. Extracted values from ICP-AES (Inductively coupled plasma atomic emission spectroscopy) measurements against the Cu:NC solution ratio for NCs with diameters of 2.7 nm (labeled A), 3.3 nm (labeled B), 3.6 nm (labeled C) and 4.1 nm (labeled D).

(7) FIG. 6 demonstrates the number of Ag atoms per NC. The number of Ag atoms per NC extracted from ICP-AES measurements against the Ag:NC solution ratio for NCs with diameters of 2.7 nm (labeled A), 3.3 nm (labeled B), 4.1 nm (labeled C) and 5.0 nm (labeled D). The inset is an enlarged representation of the region where the Ag:NC solution ratio is less than 450.

(8) FIG. 7 demonstrates the effect of doping on the STM (scanning tunneling microscopy) tunneling spectra. The effect of doping on the STM tunneling spectra is shown in four dI/dV versus V tunneling spectra at 4.2 K, of undoped (solid line), Au-doped (dash-dotted line), Cu-doped (dashed line), and Ag-doped (dotted line) InAs nanocrystals, nominally 4 nm in diameter. The doped QDs were taken from samples that had Ag, Cu, and Au atom/QD ratios corresponding to 15, 160, and 77, respectively. The vertical (V=0) dashed line is a guide to the eye, highlighting the relative shifts of the band edges in the doped samples in manner typical of p-doped and n-doped semiconductors for the Ag-doped and Cu-doped nanocrystals, respectively. The inset shows an STM image of a single (Ag-doped) QD on which STS (scanning tunneling spectroscopy) data were measured.

DETAILED DESCRIPTION OF EMBODIMENTS

(9) The invention provides a family of novel doped nanoparticles and a process for their preparation, which permits doping semiconductor nanocrystals with doping atoms, and a specific embodiment relates to metal dopants. By changing the dopant type and concentration, exquisite control of the electronic properties, including the band gap and Fermi energy is achieved. The role of strong quantum confinement leading to localization of impurity levels, as well as disorder effects leading to band-tailing in small nanocrystals, have been studied experimentally and theoretically. The successful controlled doping provides n- and p-doped semiconductor nanocrystals which greatly enhance the potential application of such materials in solar cells, thin-film transistors, and optoelectronic devices.

(10) The addition of even a single impurity atom to a semiconductor nanocrystal with a diameter of 4 nm, and which contains about 1,000 atoms, leads to a nominal doping level of 710.sup.19 atoms per cm.sup.3. In a bulk semiconductor this is already within an exceedingly highly-doped limit, where metallic (degenerate) behavior is expected. Doping at this level in bulk semiconductors leads to the several effects summarized in FIG. 1A. First, at high doping levels the impurities interact with each other and an impurity sub-band emerges near the edge of the respective band (conduction or valence for n- or p-type, respectively). Often, tail states (Urbach tail) also develop due to distortions in the crystal structure. Effectively, the band gap, E.sub.g, is narrowed. This may be probed by optical means where for highly doped n-type semiconductor (left frame in FIG. 1A) the absorption is blue shifted due to conduction band filling by the donated electrons (Moss-Burstein effect), and the emission emanating from the bottom of the conduction band is red-shifted. For high p-type doping, both absorption and emission are typically seen to be red-shifted due to the complexity of the valence band dominated by the band-tailing effect.

(11) A dramatically different situation arises for electronic impurity doping of nanocrystals due to the discrete nature of the quantum confined states (FIG. 1B, for n-type doping). In this case the addition of a single dopant (left frame) significantly alters the density of states (DOS) due to the introduction of the impurity levels, a situation which is not expected for prior remote surface doping strategies. This has been described by a hydrogenic model under spherical confinement, leading to S- and P-like impurity states denoted by E.sub.d.sup.1S and E.sub.d.sup.2P that are several tens of meVs () below the corresponding dot levels, effectively doubling the DOS near those energies. Even more intriguing is the system of the present invention, of multiple impurities in a single nanocrystal, which are inherently interacting due to the small volume and experience the effect of confining potential (FIG. 1B, right frame). Under these conditions of heavily doped semiconductor nanocrystals, the nature of delocalization and interaction of the impurity charge carriers may be greatly modified as compared to the bulk case. In addition, multiple impurities in a small confined nanocrystal can enhance disorder effects, altering the electronic structure via a quantum-confined Urbach tail mechanism.

(12) To dope InAs nanocrystals with different impurity atoms, the inventors of the present invention have modified a reaction used for gold growth onto semiconductor nanoparticles [17], demonstrating phase separation between InAs and impurity metal regions (domains). As FIG. 2A shows, for nanoparticles prepared according to the present invention, at the impurity levels recited herein, it was not possible to identify the presence of metal regions (domains). This clearly indicates that the dopant atoms were dispersed. It should be noted that this was not the case for very high metal atom concentrations (3000 atoms per QD in the reaction solution) prepared in accordance with prior processes [17], where TEM analysis clearly showed phase separation between InAs and impurity metal regions.

(13) Further support for the dispersion of impurities is provided by X-ray diffraction (data not shown), where no fingerprints of metal domains were detected while the InAs crystal structure was generally maintained. Some broadening of the peaks was observed, ascribed to a small degree of structural disorder. X-ray photoelectron spectroscopy (XPS) measurements of these samples were also performed indicating the presence of dopant atoms (Ag, Au or Cu) in the respective samples (data not shown). This suggests successful addition of these atoms to the InAs QDs. Indeed, extrapolating the diffusion parameter values to room temperature gives a diffusion length scale for Au in InAs of 10.sup.4 nm/24 h, far greater than the QD diameter, and large values are also extrapolated for Ag and Cu in InAs at room temperature.

(14) FIG. 2B shows the absorption and emission (inset) spectra of undoped and doped InAs QDs. The addition of Ag atoms resulted in a red-shift of both the first exciton absorption and the emission peaks. The addition of Cu resulted in a blue-shift of the first exciton absorption peak whereas the emission was not shifted. Addition of Au at similar concentrations did not significantly alter the observed optical gap neither in absorption nor in emission. The addition of any of these impurity atoms resulted in the gradual quenching of the emission from the QDs, yet the three impurities led to qualitatively different effects on the optical spectra and hence on the electronic properties of the doped QDs.

(15) The effect of varying amounts of impurities on the first absorption peak and on the emission is shown in FIG. 2C for InAs QDs with different diameters, FIGS. 3 and 4 represents the individual spectra. The treatment of the InAs nanocrystals with the Cu solution led to an increase in the first exciton energy (FIG. 3A-C) while the emission energy was unchanged (FIG. 3D-F). Ag treatment led to a decrease in both the first exciton energy (FIG. 4A-B) and in the emission energy (FIG. 4C-D). All metal treatments resulted in some quenching of the emission. This was totally quenched after precipitation.

(16) The amount of impurities in the QDs was estimated by the analytical method of inductively coupled plasma-atomic emission spectroscopy (ICP-AES, FIGS. 5-6). To perform this analysis, the doping reaction was carried out as described above on a range of NC sizes and at different metal precursors concentrations. The NCs were then precipitated using a non-solvent added to the solution and separated by centrifugation, dissolved using concentrated HNO.sub.3 and diluted with triply distilled water. The relative quantities of In, As, Cu, Ag and Au in these solutions was measured by ICP-AES. The amount of Cu/Ag/Au per NC was extrapolated using the ratio of In and As to Cu/Ag/Au atoms together with the estimated number of In and As atoms per NC estimated from the NC size. The results are shown for Cu and Ag in FIG. 5 and FIG. 6. These graphs represent the number of metal atoms per NC extrapolated from the ICP measurements against the metal:NC solution ratio. FIG. 5 shows the dependence of the number of Cu atoms per NC on the Cu:NC solution ratio. For each size the results were fitted and the interpolated relationship was used to calibrate the number of Cu atoms per NC reported here. The number of Ag atoms per NC on the Ag:NC solution ratio showed two regimes as can be seen in FIG. 6.

(17) A first possible source of optical spectral shifts in such quantum confined particles may be related to size changes upon doping, but this was excluded by detailed sizing analysis (data not shown). An alternative source of the spectral shifts can be associated with electronic doping by the impurities. In FIG. 7 shown are the tunneling spectra measured by a scanning tunneling microscope (STM) at T=4.2K for undoped, Au-doped, Cu-doped and Ag-doped InAs QDs 4.2 nm in diameter. Starting from the reference case of the undoped QD shown in the lower panel, the dI/dV curves, which are proportional to the DOS (density of states), match earlier studies of InAs QDs. The gap region is clearly identified, while on the positive bias side a doublet of peaks associated with tunneling through the doubly degenerate 1S.sub.e nanocrystal confined conduction band state is seen at the onset of the current, followed by higher order multiplet at higher bias corresponding to tunneling through the 1P.sub.e conduction band state. A more complex peak structure is seen on the negative bias side, resulting from tunneling through the closely spaced and intricate valence band states of the InAs nanocrystal.

(18) Several changes were seen upon doping the QDs. Starting with the case of Au, the gap was similar to the undoped NC, consistent with the optical measurements. However, the features in the scanning tunneling spectroscopy (STS) spectra were washed out, suggesting that indeed Au entered the nanocrystal, perturbing the pristine level structure. More significant changes were seen for both the Cu and Ag cases, presented in the upper panel. Significant band-tailing into the gap and emergence of in-gap states in regions covering nearly 40% of the gap region were observed. In particular, in the Cu case, a shoulder on a tail-state structure was seen at bias values just below the 1S.sub.e conduction band doublet (which is remarkably preserved). Additionally, the doublet was superimposed on a notable rising background that increased to the region of the 1P.sub.e peaks that are not well resolved. For the Ag-doped QDs, there was a significant broadening and merging of features on the positive bias side, and on the negative bias side a background signal develops.

(19) A clear result of doping in bulk semiconductors was the shift of the Fermi level, which for n-type doping was close to the conduction band, and conversely, shifts to a lower energy close to the valence band, for p-type impurities. Remarkably, such shifts were clearly identified in the STS of the Cu and Ag doped QDs measured by the positions of the band edges relative to zero bias. While the zero bias position for the undoped case, as well as the Au doped case, was nearly centered in between the valence band and conduction band onsets, in the Cu-case the onset of the conduction band states nearly merges with the zero bias position. Considering that this relative shift corresponds to a relative measure of the Fermi level of the nanoparticle, this shift is clearly indicating n-type doping in this case. In contrast, for the Ag case, the zero bias was much closer to the onset of the valence band states. Therefore the Fermi level is now close to the valence band signifying p-type doping in this case.

(20) Chemical considerations for the doping of InAs with the different metal atom impurities can help to understand these observations. Cu can have a formal oxidation state of either Cu.sup.2+/1+. Moreover, its ionic radius is the smallest of the three impurities and therefore may be accommodated in interstitial sites within the InAs lattice. In such a case, one can expect that the Cu will partly donate its valence electrons to the QD (quantum dot) leading to n-type doping, consistent with the shift in the Fermi energy observed by STS. The incorporation of multiple impurities is expected to lead to the development of closely spaced impurity states, akin to the impurity band formed in the bulk. This band forms asymmetrically due to the disordered arrangement of the impurities in the QD, surpassing the energy of the 1S.sub.e NC state. The observed rising background in the STS curve signifies the presence of such an impurity band. This is a direct indication of the substantial modification of the DOS induced by the impurities in small QDs, corresponding to very highly doped behavior in the bulk. Revisiting the observed blue shift in the absorption, this is in line with the filling of the conduction and asymmetric impurity-band levels in heavily n-type doped QD, leading to a Moss-Burstein blue-shift in the absorption spectrum and minor shifts in the emission (FIG. 2).

(21) Ag has a large radius, and is considered to be a substitutional impurity in III-V semiconductors. The replacement of an In atom, which possesses three valence electrons, with a Ag atom, which has only one valence electron, leads to an electron deficiency in the bonding orbitals causing p-type doping. This is reflected in the shift of the Fermi level, as seen in the STS data (FIG. 7, dotted trace). In this case, the rising background in the spectrum at negative bias, indicates the formation of an impurity band near the valence band. Since Ag has the largest ionic radius of the three impurities, it distorts the crystal structure most significantly; this results in band-tailing analogous to the Urbach tail known for highly doped bulk semiconductors, and leading to the red shifts observed in the absorption onset as well as the emission (FIG. 2). Au may adopt a +3 valence state, which makes it isovalent with In and hence doping is not expected to lead to the introduction of charge carriers. Moreover, its size is comparable to that of the In, allowing for substitutional doping without significant lattice distortions. These features of Au are consistent with the absence of significant shifts in absorption, emission and Fermi energy, as observed in both optical and tunneling spectra.

EXPERIMENTAL

Materials

(22) In(III)Cl.sub.3 (99.999+%), tris(trimethylsilyl) arsenide (TMS3As), trioctylphosphine (TOP, 90%; purified by vacuum distillation and kept in the glovebox), AuCl.sub.3 (99%), AgCl (99+%), AgNO.sub.3 (99+%), CuCl.sub.2 (99.999%), dodecylamine (DDA, 98%), didodecyldimethylammonium bromide (DDAB, 98%), toluene (99.8% anhydrous), methanol (99.8% anhydrous) were purchased from Sigma Aldrich except for (TMS.sub.3As) which was synthesized as detailed in the literature [17].

(23) Methods

(24) InAs Nanocrystal Synthesis

(25) The synthesis of InAs nanocrystals (NCs) was carried out under an inert atmosphere using standard Schlenck techniques.

(26) In a typical synthesis a mixture of indium and arsenic precursors were prepared by adding 0.3 g (1 mmol) of (TMS.sub.3As) to 1.7 g of a 1.4M InCl.sub.3 TOP solution (2 mmol in total). 1 ml of this solution was injected into a three neck-flask containing 2 ml of TOP at 300 C. under vigorous stirring. The temperature was then reduced to 260 C. and further precursor solution was added in order to achieve particle growth. The growth was monitored by taking the absorption spectra of aliquots extracted from the reaction solution. Upon reaching the desired size, the reaction mixture was allowed to cool to room temperature and was transferred into a glovebox. Anhydrous toluene was added to the reaction solution, and the nanocrystals were precipitated by adding anhydrous methanol. The size distribution of the nanocrystals in a typical reaction was on the order of 10%. This was improved using size selective precipitation with toluene and methanol as the solvent and anti-solvent, respectively.

(27) Metal-Atom Doping

(28) In a typical reaction a metal solution was prepared by dissolving 10 mg of the metal salt (CuCl.sub.2, AgNO.sub.3, AgCl or AuCl.sub.3), 80 mg DDAB and 120 mg of DDA in 10 ml of toluene. The Cu and Ag solutions prepared in this manner are respectively blue, colorless and yellow. The metal solution was then added drop-wise to a stirred 2 ml toluene solution of InAs NCs. After 15 minutes the absorption and emission of the solutions were measured. The Cu and Au samples were precipitated with methanol whilst the Ag sample was precipitated with acetone. The entire metal treatment procedure was carried out under inert conditions. The ratio of metal atoms to NCs in solution was estimated from the literature values of InAs NC absorption cross-sections.