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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Morphology control of cadmium selenide nanocrystals: insights into the roles of di-n-octylphosphine oxide (DOPO) and ucid (DOPA).

Di-n-octylphosphine oxide (DOPO) and di-n-octylphosphinic acid (DOPA), as two of impurities found in commercial tri-n-octylphosphine oxide (TOPO), generate significant differences in the outcomes of CdSe-nanocrystal (NC) syntheses. Using n-tetradecylphosphonic acid (TDPA) as the primary acid additive, quantum dots (QDs) are grown with DOPO added, whereas quantum rods (QRs) are grown in the presence of DOPA. While using oleic acid (OA) as the primary acid additive, QDs are generated and the QDs produced with DOPA exhibit larger sizes and size distributions than those produced with DOPO. (31)P NMR analyses of the reaction mixtures reveal that the majority of the DOPO has been converted into DOPA and di-n-octylphosphine (DOP) with DOP being removed via evacuation over the course of Cd-precursor preparation. The origin of the puzzling differences in the shape control of CdSe NCs in the presence of DOPO and DOPA is elucidated to be the small quantity of DOPO present, which liberates DOP during NC synthesis. In the presence of DOP, regardless of DOPA, the precursor-conversion kinetics and thus the nucleation kinetics are dramatically accelerated, generating a large number of nuclei by consuming a significant amount of CdSe nutrients, favoring QD growth. Similarly, QD growth is favored by the fast nucleation kinetics in the presence of OA, and the broader size distributions of QDs with DOPA are due to a second nucleation event initiated by the more stable Cd-di-n-octylphosphinate component. In contrast, a slow nucleation event results in the growth of QRs in the case of using DOPA and TDPA, where no DOPO or DOP is present. The results, thus, demonstrate the important role of precursor-conversion kinetics in the control of NC morphologies.

Bright core-shell semiconductor quantum wires.

Colloidal CdTe quantum wires are reported having ensemble photoluminescence efficiencies as high as 25% under low excitation-power densities. High photoluminescence efficiencies are achieved by formation of a monolayer CdS shell on the CdTe quantum wires. Like other semiconductor nanowires, the CdTe quantum wires may contain frequent wurtzite-zinc-blende structural alternations along their lengths. The present results demonstrate that the optical properties, emission-peak shape and photoluminescence efficiencies, are independent of the presence or absence of such structural alternations.

Lamellar assembly of cadmium selenide nanoclusters into quantum belts.

Here, we elucidate a double-lamellar-template pathway for the formation of CdSe quantum belts. The lamellar templates form initially by dissolution of the CdX(2) precursors in the n-octylamine solvent. Exposure of the precursor templates to selenourea at room temperature ultimately affords (CdSe)(13) nanoclusters entrained within the double-lamellar templates. Upon heating, the nanoclusters are transformed to CdSe quantum belts having widths, lengths, and thicknesses that are predetermined by the dimensions within the templates. This template synthesis is responsible for the excellent optical properties exhibited by the quantum belts. We propose that the templated-growth pathway is responsible for the formation of the various flat, colloidal nanocrystals recently discovered, including nanoribbons, nanoplatelets, nanosheets, and nanodisks.

Origin of high photoluminescence efficiencies in CdSe quantum belts.

CdSe quantum belts (QBs) having lengths of 0.5-1.5 microm and thicknesses of 1.5-2.0 nm exhibit high photoluminescence (PL) efficiencies of approximately 30%. Epifluorescence studies establish the PL spectra to be uniform along single QBs, and nearly the same from QB to QB. Photogenerated excitons are shown to be effectively delocalized over the entire QBs by position-selective excitation. Decoration of the QBs with gold nanoparticles indicates a low density of surface-trap sites, located primarily on the thin belt edges. The high PL efficiencies and effective exciton delocalization are attributed to the minimization of defective {1100} edge surface area or edge-top/bottom (face) line junctions in QBs relative to quantum wires having roughly isotropic cross sections, for which very low PL quantum efficiencies have been reported. The results suggest that trap sites can be minimized in pseudo-one-dimensional nanocrystals, such that the facile transport of energy and charge along their long axes becomes possible.

An easy shortcut synthesis of size-controlled bismuth nanoparticles and their use in the SLS growth of high-quality colloidal cadmium selenide quantum wires.

An easy shortcut synthesis of thermally stable, near-monodisperse Bi nanoparticles from BiCl(3) and Na[N(SiMe(3))(2)] is described. The diameters of the Bi nanoparticles are controlled in the range of 4-29 nm by varying the amounts of BiCl(3) and Na[N(SiMe(3))(2)] employed. Standard deviations in their diameter distributions are 5-15% of the mean diameters, consistent with near monodispersity. These Bi nanoparticles are shown to be the best currently available catalysts for the solution-liquid-solid (SLS) growth of high-quality CdSe quantum wires.

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.

Spectroscopic identification of tri-n-octylphosphine oxide (TOPO) impurities and elucidation of their roles in cadmium selenide quantum-wire growth.

Tri-n-octylphosphine oxide (TOPO) is the most commonly used solvent for the synthesis of colloidal nanocrystals. Here we show that the use of different batches of commercially obtained TOPO solvent introduces significant variability into the outcomes of CdSe quantum-wire syntheses. This irreproducibility is attributed to varying amounts of phosphorus-containing impurities in the different TOPO batches. We employ (31)P NMR to identify 10 of the common TOPO impurities. Their beneficial, harmful, or negligible effects on quantum-wire growth are determined. The impurity di-n-octylphosphinic acid (DOPA) is found to be the important beneficial TOPO impurity for the reproducible growth of high-quality CdSe quantum wires. DOPA is shown to beneficially modify precursor reactivity through ligand substitution. The other significant TOPO impurities are ranked according to their abilities to similarly influence precursor reactivity. The results are likely of general relevance to most nanocrystal syntheses conducted in TOPO.

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.

Synthesis of cadmium telluride quantum wires and the similarity of their effective band gaps to those of equidiameter cadmium telluride quantum dots.

High-quality colloidal CdTe quantum wires having purposefully controlled diameters in the range 5-11 nm are grown by the solution-liquid-solid (SLS) method, using Bi nanoparticle catalysts, cadmium octadecylphosphonate and trioctylphosphine telluride as precursors, and a TOPO solvent. The wires adopt the wurtzite structure and grow along the [002] direction (parallel to the c axis). The size dependence of the effective band gaps in the wires is determined from the absorption spectra and compared to the experimental results for high-quality CdTe quantum dots. In contrast to the predictions of an effective-mass approximation, particle-in-a-box model, and previous experimental results from CdSe and InP dot-wire comparisons, the effective band gaps of CdTe dots and wires of like diameter are found to be experimentally indistinguishable. The present results are analyzed using density functional theory under the local-density approximation by implementing a charge-patching method. The higher-level theoretical analysis finds the general existence of a threshold diameter, above which dot and wire effective band gaps converge. The origin and magnitude of this threshold diameter are discussed.