Bound-Ion Pair X-Type Ligation of Cadmium and Zinc Dithiocarbamates on Cadmium Selenide Quantum Belts.

Wurtzite CdSe quantum belts with L-type n-octylamine, L-type ammonia, or Z-type Cd(oleate)2 ligands are exchanged for several metal-dithiocarbamate ligands [M(S2CNR1R2)2]: Cd(S2CNPhMe)2, Cd(S2CNEt2)2, Zn(S2CNPhMe)2, and Zn(S2CNEt2)2. Successful ligand exchange with all M(S2CNR1R2)2 compounds occurs from {CdSe[Cd(oleate)2]0.19} quantum belts (QBs), which induce similar spectral shifts in the absorption spectra of the ligand-exchanged QBs. Spectroscopic data, experimentally determined lattice strains, and ligand exchanges with [Na][Et2NCS2] and [NH4][MePhNCS2] establish that the [M(S2CNR1R2)2] ligands bind as bound-ion-paired X-type ligands with (S2CNR1R2)- groups ligated directly to the QB surfaces and [M(S2CNR1R2)]+ groups serving as the charge-balancing ion-paired countercations. The X-type dithiocarbamate ligands do not impart any special electronic effects to the CdSe QBs.

Intraband Relaxation Dynamics of Charge Carriers within CdTe Quantum Wires.

The state-to-state intraband relaxation dynamics of charge carriers photogenerated within CdTe quantum wires (QWs) are characterized via transient absorption spectroscopy. Overlapping signals from the energetic-shifting of the quantum-confinement features and the occupancy of carriers in the states associated with these features are separated using the quantum-state renormalization model. Holes generated with an excitation energy of 2.75 eV reach the band edge within the instrument response of the measurement, ∼200 fs. This extremely short relaxation time is consistent with the low photoluminescence quantum yield of the QWs, ∼0.2%, and the presence of alternative relaxation pathways for the holes. The electrons relax through the different energetically available quantum-confinement states, likely via phonon coupling, with an overall rate of ∼0.6 eV ps-1.

Excitation Energy Dependence of Photoluminescence Quantum Yields in Semiconductor Nanomaterials with Varying Dimensionalities.

The excitation energy dependence (EED) of the photoluminescence quantum yield (ΦPL) of semiconductor nanoparticles with varying dimensionalities is reported. Specifically, the EEDs of CdSe quantum dots, CdSe quantum platelets, CdSe quantum belts, and CdTe quantum wires were determined via measurements of individual ΦPL values and photoluminescence efficiency (PLEff(E)) spectra. There is a general trend of overall decreasing efficiency for radiative recombination with increasing excitation energy. In addition, there are often local minima in the PLEff(E) spectra that are most often at energies between quantum-confinement transitions. The average PL lifetimes of the samples do not depend on the excitation energy, suggesting that the EED of ΦPL arises from charge carrier trapping that competes efficiently with intraband carrier relaxation to the band edge. The local minima in the PLEff(E) spectra are attributed to excitation into optically coupled states that results in the loss of carriers in the semiconductor. The EED data suggest that the PLEff(E) spectra depend on the sample synthesis, preparation, surface passivation, and environment.

Core-Shell Cadmium Telluride Quantum Platelets with Absorptions Spanning the Visible Spectrum.

CdS and CdSe shells are deposited on wurtzite CdTe quantum platelets (nanoplatelets) by exchanging the initial primary-amine ligation to Cd(OAc)2 ligation, with subsequent reaction of the Cd(OAc)2 ligand shell and thiourea or selenourea, respectively. Shell deposition is conducted in a cyclic manner, with 0.21-0.34 monolayers of CdS and 0.99-1.20 monolayers of CdSe being deposited in each cycle. The CdTe quantum platelets having an initial thickness of 1.9 nm are converted to CdTe-CdS and CdTe-CdSe core-shell quantum platelets having maximum thicknesses of 3.0 and 6.3 nm, respectively. The morphologies and wurtzite structure of the initial CdTe quantum platelets are retained upon shell deposition. The absorption spectrum of the CdTe quantum platelets is progressively shifted to lower energy with increasing shell thickness, across the entire visible spectrum. The spectral shifts observed scale with the inverse square of the total core-shell thickness.

Isolation of Amine Derivatives of (ZnSe)34 and (CdTe)34. Spectroscopic Comparisons of the (II-VI)13 and (II-VI)34 Magic-Size Nanoclusters.

The spectroscopically observed magic-size nanoclusters (ZnSe)34 and (CdTe)34 are isolated as amine derivatives. The nanoclusters [(ZnSe)34( n-octylamine)29±6(di- n-octylamine)5±4] and [(CdTe)34( n-octylamine)4±3(di- n-pentylamine)13±3] are fully characterized by combustion-based elemental analysis, UV-visible spectroscopy, IR spectroscopy, and mass spectrometry. Amine derivatives of both (ZnSe)34 and (CdTe)34 are observed to convert to the corresponding (ZnSe)13 and (CdTe)13 derivatives, indicating that the former are kinetic products and the latter thermodynamic products, under the conditions employed. This conversion process is significantly inhibited in the presence of secondary amines. The isolation of the two new nanocluster derivatives adds to a total of nine of 12 possible isolated derivatives in the (II-VI)13 and (II-VI)34 families (II = Zn, Cd; VI = S, Se, Te), allowing comparisons of their properties. The members of these two families exhibit extensive spectroscopic homologies. In both the (II-VI)13 and (II-VI)34 families, linear relationships are established between the lowest-energy nanocluster electronic transition and the band gap of the corresponding bulk semiconductor phase.

Tellurium Precursor for Nanocrystal Synthesis: Tris(dimethylamino)phosphine Telluride.

Preparations of CdTe quantum platelets, magic-size (CdTe)13 nanoclusters, and CdTe quantum wires are described using (Me2N)3PTe (with (Me2N)3P) as a Te precursor. The (Me2N)3PTe/(Me2N)3P precursor mixture is shown to be more reactive than mixtures of trialkylphosphine tellurides and the corresponding trialkylphosphines, R3PTe/R3P, which are commonly employed in nanocrystal syntheses. For syntheses conducted in primary amine solvents, (Me2N)3PTe and (Me2N)3P undergo a transamination reaction, affording (Me2N) x(RHN)3- xPTe and (Me2N) x(RHN)3- xP (R = n-octyl or oleyl). The transaminated (Me2N) x(RHN)3- xPTe derivatives are shown to be the likely Te precursors under those conditions. The enhanced reactivities of the tris(amino)phosphine tellurides are ascribed to increased nucleophilicity due to the amino-N lone pairs.

Wave Function Engineering in CdSe/PbS Core/Shell Quantum Dots.

The synthesis of epitaxial CdSe/PbS core/shell quantum dots (QDs) is reported. The PbS shell grows in a rock salt structure on the zinc blende CdSe core, thereby creating a crystal structure mismatch through additive growth. Absorption and photoluminescence (PL) band edge features shift to lower energies with increasing shell thickness, but remain above the CdSe bulk band gap. Nevertheless, the profiles of the absorption spectra vary with shell growth, indicating that the overlap of the electron and hole wave functions is changing significantly. This leads to over an order of magnitude reduction of absorption near the band gap and a large, tunable energy shift, of up to 550 meV, between the onset of strong absorption and the band edge PL. While the bulk valence and conduction bands adopt an inverse type-I alignment, the observed spectroscopic behavior is consistent with a transition between quasi-type-I and quasi-type-II behavior depending on shell thickness. Three effective mass approximation models support this hypothesis and suggest that the large difference in effective masses between the core and shell results in hole localization in the CdSe core and a delocalization of the electron across the entire QD. These results show the tuning of wave functions and transition energies in CdSe/PbS nanoheterostructures with prospects for use in optoelectronic devices for luminescent solar concentration or multiexciton generation.

Role of Precursor-Conversion Chemistry in the Crystal-Phase Control of Catalytically Grown Colloidal Semiconductor Quantum Wires.

Crystal-phase control is one of the most challenging problems in nanowire growth. We demonstrate that, in the solution-phase catalyzed growth of colloidal cadmium telluride (CdTe) quantum wires (QWs), the crystal phase can be controlled by manipulating the reaction chemistry of the Cd precursors and tri-n-octylphosphine telluride (TOPTe) to favor the production of either a CdTe solute or Te, which consequently determines the composition and (liquid or solid) state of the BixCdyTez catalyst nanoparticles. Growth of single-phase (e.g., wurtzite) QWs is achieved only from solid catalysts (y ≪ z) that enable the solution-solid-solid growth of the QWs, whereas the liquid catalysts (y ≈ z) fulfill the solution-liquid-solid growth of the polytypic QWs. Factors that affect the precursor-conversion chemistry are systematically accounted for, which are correlated with a kinetic study of the composition and state of the catalyst nanoparticles to understand the mechanism. This work reveals the role of the precursor-reaction chemistry in the crystal-phase control of catalytically grown colloidal QWs, opening the possibility of growing phase-pure QWs of other compositions.

Cadmium Bis(phenyldithiocarbamate) as a Nanocrystal Shell-Growth Precursor.

Cadmium bis(phenyldithiocarbamate) [Cd(PTC)2] is prepared and structurally characterized. The compound crystallizes in the monoclinic space group P21/n. A one-dimensional polymeric structure is adopted in the solid state, having bridging PTC ligands and 6-coordinate pseudo-octahedral Cd atoms. The compound is soluble in DMSO, THF, and DMF and insoluble in EtOH, MeOH, CHCl3, CH2Cl2, and toluene. {CdSe[n-octylamine]0.53} quantum belts and Cd(PTC)2 react to deposit epitaxial CdS shells on the nanocrystals. With an excess of Cd(PTC)2, the resulting thick shells contain spiny CdS nodules grown in the Stranski-Krastanov mode. Stoichiometric control affords smooth, monolayer CdS shells. A base-catalyzed reaction pathway is elucidated for the conversion of Cd(PTC)2 to CdS, which includes phenylisothiocyanate and aniline as intermediates, and 1,3-diphenylthiourea as a final product.

Reversible Exchange of L-Type and Bound-Ion-Pair X-Type Ligation on Cadmium Selenide Quantum Belts.

CdSe quantum belts of composition {CdSe[n-octylamine]0.53} and protic acids HX (X = Cl, Br, NO3, acetate (OAc), and benzoate (OBz)) react to exchange the L-type amine ligation to bound-ion-pair X-type ligation. The latter ligation has X- anions bound to the nanocrystal surfaces and closely associated LH+ counter-cations (protonated n-octylamine or tri-n-octylphosphine (TOP) to balance the surface charges. The compositions of the exchanged QBs are {CdSe[Br]0.44[n-octylammonium]0.41}, {CdSe[NO3]0.10[TOPH]0.12}, {CdSe[OBz]0.08[n-octylammonium]0.02[TOPH]0.06}, and {CdSe[OAc]0.16[n-octylammonium]0.02[TOPH]0.14}. (The HCl-exchanged QBs are insufficiently stable for elemental analysis.) The bound-ion-pair X-type ligation is fully reversed to L-type n-octylamine ligation in the cases of X = NO3, acetate, and benzoate. The ligand exchanges are monitored by absorption spectroscopy, and the exchanged, bound-ion-pair X-type ligated nanocrystals are characterized by a range of methods.

Spectroscopic Properties of Phase-Pure and Polytypic Colloidal Semiconductor Quantum Wires.

We report ensemble extinction and photoluminesence spectra for colloidal CdTe quantum wires (QWs) with nearly phase-pure, defect-free wurtzite (WZ) structure, having spectral line widths comparable to the best ensemble or single quantum-dot values, to the single polytypic (having WZ and zinc blende (ZB) alternations) QW values, and to those of two-dimensional quantum belts or platelets. The electronic structures determined from the multifeatured extinction spectra are in excellent agreement with the theoretical results of WZ QWs having the same crystallographic orientation. Optical properties of polytypic QWs of like diameter and diameter distribution are provided for comparison, which exhibit smaller bandgaps and broader spectral line widths. The nonperiodic WZ-ZB alternations are found to generate non-negligible shifts of the bandgap to intermediate energies between the quantum-confined WZ and ZB energies. The alternations and variations in the domain sizes result in inhomogeneous spectral line width broadening that may be more significant than that arising from the 12-13% diameter distributions within the QW ensembles.

Solution-Liquid-Solid Synthesis, Properties, and Applications of One-Dimensional Colloidal Semiconductor Nanorods and Nanowires.

The solution-liquid-solid (SLS) and related solution-based methods for the synthesis of semiconductor nanowires and nanorods are reviewed. Since its discovery in 1995, the SLS mechanism and its close variants have provided a nearly general strategy for the growth of pseudo-one-dimensional nanocrystals. The various metallic-catalyst nanoparticles employed are summarized, as are the syntheses of III-V, II-VI, IV-VI, group IV, ternary, and other nanorods and nanowires. The formation of axial heterojunctions, core/shell nanowires, and doping are also described. The related supercritical-fluid-liquid-solid (SFLS), electrically controlled SLS, flow-based SLS, and solution-solid-solid (SSS) methods are discussed, and the crystallographic characteristics of the wires and rods grown by these methods are summarized. The presentation of optical and electronic properties emphasizes electronic structures, absorption cross sections, polarization anisotropies, and charge-carrier dynamics, including photoluminescence intermittency (blinking) and photoluminescence modulation by charges and electric fields. Finally, developing applications for the pseudo-one-dimensional nanostructures in field-effect transistors, lithium-ion batteries, photocathodes, photovoltaics, and photodetection are discussed.

Crystal-Phase Control by Solution-Solid-Solid Growth of II-VI Quantum Wires.

A simple and potentially general means of eliminating the planar defects and phase alternations that typically accompany the growth of semiconductor nanowires by catalyzed methods is reported. Nearly phase-pure, defect-free wurtzite II-VI semiconductor quantum wires are grown from solid rather than liquid catalyst nanoparticles. The solid-catalyst nanoparticles are morphologically stable during growth, which minimizes the spontaneous fluctuations in nucleation barriers between zinc blende and wurtzite phases that are responsible for the defect formation and phase alternations. Growth of single-phase (in our cases the wurtzite phase) nanowires is thus favored.

Large Exciton Energy Shifts by Reversible Surface Exchange in 2D II-VI Nanocrystals.

Reaction of n-octylamine-passivated {CdSe[n-octylamine](0.53±0.06)} quantum belts with anhydrous metal carboxylates M(oleate)2 (M = Cd, Zn) results in a rapid exchange of the L-type amine passivation for Z-type M(oleate)2 passivation. The cadmium-carboxylate derivative is determined to have the composition {CdSe[Cd(oleate)2](0.19±0.02)}. The morphologies and crystal structures of the quantum belts are largely unaffected by the exchange processes. Addition of n-octylamine or oleylamine to the M(oleate)2-passivated quantum belts removes M(oleate)2 and restores the L-type amine passivation. Analogous, reversible surface exchanges are also demonstrated for CdS quantum platelets. The absorption and emission spectra of the quantum belts and platelets are reversibly shifted to lower energy by M(oleate)2 passivation vs amine passivation. The largest shift of 140 meV is observed for the Cd(oleate)2-passivated CdSe quantum belts. These shifts are attributed entirely to changes in the strain states in the Zn(oleate)2-passivated nanocrystals, whereas changes in strain states and confinement dimensions contribute roughly equally to the shifts in the Cd(oleate)2-passivated nanocrystals. Addition of Cd(oleate)2, which electronically couples to the nanocrystal lattices, increases the effective thickness of the belts and platelets by approximately a half of a monolayer, thus increasing the confinement dimension.

Influence of the Nanoscale Kirkendall Effect on the Morphology of Copper Indium Disulfide Nanoplatelets Synthesized by Ion Exchange.

CuInS2 nanocrystals are prepared by ion exchange with template Cu2-xS nanoplatelets and InX3 [X = chloride, iodide, acetate (OAc), or acetylacetonate (acac)]. The morphologies of the resultant nanocrystals depend on the InX3 precursor and the reaction temperature. Exchange with InCl3 at 150 °C produces CuInS2 nanoplatelets having central holes and thickness variations, whereas the exchange at 200 °C produces intact CuInS2 nanoplatelets in which the initial morphology is preserved. Exchange with InI3 at 150 °C produces CuInS2 nanoplatelets in which the central hollowing is more extreme, whereas exchange with In(OAc)3 or In(acac)3 at 150 °C produces intact CuInS2 nanoplatelets. The results establish that the ion exchange occurs through the thin nanoplatelet edge facets. The hollowing and hole formation are due to a nanoscale Kirkendall Effect operating in the reaction-limited regime for displacement of X(-) at the edges, to allow insertion of In(3+) into the template nanoplatelets.

Magic-size II-VI nanoclusters as synthons for flat colloidal nanocrystals.

Five new, discretely sized, magic-size II-VI nanoclusters are synthesized in primary-amine bilayer templates and are isolated as the derivatives [(CdS)(34)(n-butylamine)(18)], [(ZnS)(34)(n-butylamine)(34)], [(ZnSe)(13)(n-butylamine)(13)], [(CdTe)(13)(n-propylamine)(13)], and [(ZnTe)(13)(n-butylamine)(13)]. The nanoclusters are characterized by elemental analysis, UV-visible absorption spectroscopy, laser-desorption-ionization mass spectrometry, and transmission electron microscopy. Four of the nanocluster precursors are converted to wurtzitic CdS, ZnS, and ZnSe quantum platelets and CdTe quantum belts, respectively, under mild conditions.

Two-dimensional semiconductor nanocrystals: properties, templated formation, and magic-size nanocluster intermediates.

CONSPECTUS: Semiconductor nanocrystals having an extended length dimension and capable of efficiently transporting energy and charge would have useful applications in solar-energy conversion and other emerging technologies. Pseudocylindrical semiconductor nanowires and quantum wires are available that could potentially serve in this role. Sadly, however, their defective surfaces contain significant populations of surface trap sites that preclude efficient transport. The very large surface area of long wires is at least part of the problem. As electrons, holes, and excitons migrate along a nanowire or quantum wire, they are exposed to an extensive surface and to potentially large numbers of trap sites. A solution to this dilemma might be found by identifying "long" semiconductor nanocrystals of other morphologies that are better passivated. In this Account, we discuss a newly emerging family of flat semiconductor nanocrystals that have surprising characteristics. These thin, flat nanocrystals have up to micrometer-scale (orthogonal) lateral dimensions and thus very large surface areas. Even so, their typical photoluminescence efficiencies of 30% are astonishingly high and are 2 orders of magnitude higher than those typical of semiconductor quantum wires. The very sharp emission spectra of the pseudo-two-dimensional nanocrystals reflect a remarkable uniformity in their discrete thicknesses. Evidence that excitons are effectively delocalized and hence transported over the full dimensions of these nanocrystals has been obtained. The excellent optical properties of the flat semiconductor nanocrystals confirm that they are exceptionally well passivated. This Account summarizes the two synthetic methods that have been developed for the preparation of pseudo-two-dimensional semiconductor nanocrystals. A discussion of their structural features accounts for their discrete, uniform thicknesses and details the crystal-lattice expansions and contractions they exhibit. An analysis of their optical properties justifies the sharp photoluminescence spectra and high photoluminescence efficiencies. Finally, a bilayer mesophase template pathway is elucidated for the formation of the nanocrystals, explaining their flat morphologies. Magic-size nanocluster intermediates are found to be potent nanocrystal nucleants, allowing the synthesis temperatures to be decreased to as low as room temperature. The potential of these flat semiconductor nanocrystals in the form of nanoribbons or nanosheets for long-range energy and charge transport appears to be high.

The Magic-Size Nanocluster (CdSe)34 as a Low-Temperature Nucleant for Cadmium Selenide Nanocrystals; Room-Temperature Growth of Crystalline Quantum Platelets.

Reaction of Cd(OAc)2·2H2O and selenourea in primary-amine/secondary-amine cosolvent mixtures affords crystalline CdSe quantum platelets at room temperature. Their crystallinity is established by X-ray diffraction analysis (XRD), high-resolution transmission electron microscopy (TEM), and their sharp extinction and photoluminescence spectra. Reaction monitoring establishes the magic-size nanocluster (CdSe)34 to be a key intermediate in the growth process, which converts to CdSe quantum platelets by first-order kinetics with no induction period. The results are interpreted to indicate that the critical crystal-nucleus size for CdSe under these conditions is in the range of (CdSe)34 to (CdSe)68. The nanocluster is obtained in isolated form as [(CdSe)34(n-octylamine)16(di-n-pentylamine)2], which is proposed to function as crystal nuclei that may be stored in a bottle.

Silver chloride as a heterogeneous nucleant for the growth of silver nanowires.

Various additives are employed in the polyol synthesis of silver nanowires (Ag NWs), which are typically halide salts such as NaCl. A variety of mechanistic roles have been suggested for these additives. We now show that the early addition of NaCl in the polyol synthesis of Ag NWs from AgNO3 in ethylene glycol results in the rapid formation of AgCl nanocubes, which induce the heterogeneous nucleation of metallic Ag upon their surfaces. Ag NWs subsequently grow from these nucleation sites. The conclusions are supported by studies using ex situ generated AgCl nanocubes.

Preparation of primary amine derivatives of the magic-size nanocluster (CdSe)13.

Four [(CdSe)13(RNH2)13] derivatives (R = n-propyl, n-pentyl, n-octyl, and oleyl) are prepared by reaction of Cd(OAc)2·2H2O and selenourea in the corresponding primary-amine solvent. Nanoclusters grow in spontaneously formed amine-bilayer templates and are characterized by elemental analysis, IR spectroscopy, UV-vis spectroscopy, TEM, and low-angle XRD. Derivative [(CdSe)13(n-propylamine)13] is isolated as a yellowish-white solid (MP 98 °C) on the gram scale. These compounds are the first derivatives of magic-size CdSe nanoclusters to be isolated in purity.