Larger Nanocrystals Research Articles

Wannier function analysis of InP nanocrystals under pressure

The effect of quantum confinement upon the pressure induced direct to indirect interband transition in InP nanocrystals is investigated. We examine the behavior of the transition as a function of nanocrystal size. It is computed that smaller nanocrystals require less pressure to become indirect than larger nanocrystals. Our results suggest that the properties of the transition may be sensitive to nanocrystal stoichiometry.

Optical properties of CdSe nanocrystals in a polymer matrix

Cadmium selenide nanocrystals have been made in a transparent polymer matrix by synthesis at room temperature. Particle size is controlled by reaction time or reagent concentration. Larger nanocrystals (≳3 nm) show near-band-edge photoluminescence. Smaller ones have a broad emission extending across the visible region. Atmospheric moisture causes the emission intensity to decrease but subsequent exposure to light reverses this and gives a material which is no longer quenched by moisture. The broad emission and stability against atmospheric moisture leads to possible device applications of a “white” phosphor.

Coherent transient phenomena of confined excitons in CuCl nanocrsytals: non-Markovian dephasing and quantum beats

We have investigated coherent transient phenomena of confined excitons in CuCl nanocrystals with mean radii R in the range 2.7–23 nm embedded in glass by means of femtosecond degenerated four-wave mixing (DFWM). In the transient DFWM signal for R<5 nm, the non-Markovian behavior is observed, and the analysis based on the stochastic model yields the reservoir correlation time of 4.4–8.5 ps. The size dependence of non-Markovian behavior is explained by an increase of exciton–acoustic phonon coupling strength and a change in the acoustic phonon density of states due to quantum confinement of acoustic phonons. In the spectrally resolved DFWM signal, oscillating behaviors have been observed. In large nanocrystals with R>6.5 nm, quantum beats between the confined exciton sublevels have been observed, while for R<6.5 nm, we have observed quantum beats between the confined exciton and LO-phonon-assisted transitions due to the formation of exciton–LO-phonon complexes.

Depth distribution of luminescent Si nanocrystals in Si implanted SiO2 films on Si

Depth-resolved measurements of the photoluminescence of Si implanted and annealed SiO2 films on Si have been performed to determine the depth distribution of luminescent Si nanocrystals. Si nanocrystals with diameters ranging from ∼2 to 5 nm were formed by implantation of 35 keV Si ions into a 110-nm-thick thermally grown SiO2 film on Si(100) at a fluence of 6×1016 Si/cm2, followed by a thermal anneal at 1100 °C for 10 min. The photoluminescence spectrum is broad, peaks at λ=790 nm, and contains contributions from both recombination of quantum confined excitons in the nanocrystals and ion-implantation-induced defects. By chemical etching through the SiO2 film in steps and analyzing the changes in the photoluminescence spectrum after each etch step, the depth from which each of the two luminescence features originate is determined. The etch rate of the oxide, as derived from Rutherford backscattering spectrometry data, varies from 1.3 nm/s in the regions of small excess Si to 0.6 nm/s at the peak of the concentration profile (15 at. % excess Si). It is found that the defect luminescence originates from an ∼15-nm-thick near-surface region. Large nanocrystals luminescing at long wavelengths (λ=900 nm) are mainly located in the center of the film, where the Si concentration is highest (48 at. %). This is corroborated by transmission electron microscopy that shows a high density of Si nanocrystals in the size range of 2–5 nm in the center of the film. The largest density of small luminescent nanocrystals (λ=700 nm), not detectable by electron microscopy is found near the SiO2 surface and the SiO2/Si interface. This is attributed to either the fact that the surface and the SiO2/Si interface affect the Si nanocrystal nucleation kinetics in such a way that small nanocrystals are preferentially formed in these regions, or an optical interaction between nanocrystals of different sizes that quenches the luminescence of small nanocrystals in the center of the film.

Quantum beats of confined excitons in CuCl nanocrystals

We have performed femtosecond degenerate four-wave-mixing (DFWM) to investigate coherent transient phenomena in CuCl nanocrystals. We present observation of quantum coherence of weakly confined excitons in nanocrystals through quantum beats in the spectrally resolved signal of DFWM. In large nanocrystals with a mean radius R of 220 Å. the quantum beat between the sublevels of confined excitons is observed. In small nanocrystals with R = 40 A ̊ , we observe the quantum beat between the confined exciton and its optical phonon side band. These results suggest that the coupling of confined excitons to optical phonons becomes stronger for the smaller sizes. In the decay behavior of coherence, the two-component decay is observed, and the fast and slow components are attributed to the homogeneous width of the phonon side band and that of zero-phonon band of the confined exciton transition, respectively.

Microstructure and Size Distribution of Compound Semiconductor Nanocrystals Synthesized by Ion Implantation

AbstractIon implantation is a versatile technique by which compound semiconductor nanocrystals may be synthesized in a wide variety of host materials. The component elements that form the compound of interest are implanted sequentially into the host, and nanocrystalline precipitates then form during thermal annealing. Using this technique, we have synthesized compound semiconductor nanocrystal precipitates of ZnS, CdS, PbS, and CdSe in a fused silica matrix. The resulting microstructures and size distributions were investigated by cross-sectional transmission electron microscopy. Several unusual microstructures were observed, including a band of relatively large nanocrystals at the end of the implant profile for ZnS and CdSe, polycrystalline agglomerates of a new phase such as γ-Zn 2SiO4, and the formation of central voids inside CdS nanocrystals. While each of these microstructures is of fundamental interest, such structures are generally not desirable for potential device applications for which a uniform, monodispersed array of nanocrystals is required. Methods were investigated by which these unusual microstructures could be eliminated.

Chemical Solution Deposited Copper-doped CdSe Quantum Dot Films

AbstractCopper doped CdSe quantum dots (QDs) and larger nanocrystals were deposited by chemical solution deposition at two different temperatures (5°C and 80°C) and their morphological, optical and surface properties compared with similarly deposited undoped CdSe. Crystal sizes and structure were investigated by X-ray diffraction and transmission electron microscopy. All the samples exhibited cubic (sphalerite) structure except for the 80°C Cu-doped ones which were substantially hexagonal (wurtzite). For the 5°C samples, the crystal sizes were similar for the doped and undoped samples (ca. 3.4 nm). For the 80°C samples, the crystal size of the doped CdSe was much larger (84 nm) than the undoped one (6.3 nm) and extensive twinning occurred. Optical measurements on the doped samples showed the presence of surface states due to the copper seen as an increase in absorbance in the long wavelength region. Xray photoemission spectroscopy similarly showed tails in the valence band spectra of the doped samples. These tails were dominant in the 5°C samples and less so for the 80°C ones. XPS also showed that the Cu was present predominantly in the univalent state. The copper was found to be located on the QD surface for the 2–3 nm QDs and distributed both on the surface and in the bulk (or at twin boundaries) for the larger nanoparticles.

Spectral hole-burning and hole-filling in CuBr nanocrystals

Transient and persistent spectral hole-burning (SHB) and hole-filling phenomena are studied in nanometer-size CuBr crystals embedded in the borosilicate glass matrix. Pulsed dye lasers are used to measure SHB in the inhomogeneously broadened excitonic absorption bands at low temperatures. While large nanocrystals show only overall changes of absorption bands, in small nanocrystals (mean radius < 4 nm) relatively narrow spectral holes are generated by a selective excitation. SHB is studied at different photon energies and at different temperatures from 2 to 180 K. By thermal effects or under excitation at photon energies different from that of the spectral hole, burned holes can be erased. The spontaneous hole-filling (HF), thermally-induced HF and light-induced HF experiments give additional information about the persistent SHB mechanism and consequently about nanocrystal-matrix interactions.

Electron Dynamics of Passivated Gold Nanocrystals Probed by Subpicosecond Transient Absorption Spectroscopy

The electronic dynamics of gold nanocrystals, passivated by a monolayer of alkylthiol(ate) groups, were studied by transient spectroscopy after excitation with subpicosecond laser pulses. Three solution-phase gold samples with average particle size of 1.9, 2.6, and 3.2 nm with size distribution less than 10% were used. The photoexcitation in the intraband (surface plasmon region) leads to the heating of the conduction electron gas and its subsequent thermalization through electron−electron and electron−phonon interaction. The results are analyzed in terms of the contribution of the equilibrated “hot” electrons to the surface plasmon resonance of gold. A different spectral response was observed for different sizes of gold nanoparticles. The results were compared to the dynamics of the large (30 nm diameter) gold nanocrystals colloidal solution. The size-dependent spectral changes are attributed to the reduction of the density of states for small nanoparticles. The observed variation in the kinetics of the ...

Metastable DenseSemiconductor Phases

The creation of new materials may be possible by searching out high-energy forms of solid-state structures. In his Perspective, Brus discusses results reported in the same issue by Chenet al. on direct measurements of structural changes in nanocrystals. The measurements show that metastable higher-energy forms are more stable in larger nanocrystals, suggesting that it may be possible to trap these useful but otherwise unstable structures.

Photoluminescence mechanism in surface-oxidized silicon nanocrystals

We have studied photoluminescence (PL) properties of surface-oxidized Si nanocrystals fabricated with a ${\mathrm{SiH}}_{4}$ plasma cell. The size dependence of the PL spectrum, the PL decay dynamics, and site-selective excitation spectroscopy show that the efficient and broad PL band around \ensuremath{\sim}1.65 eV originates from excitons localized at the interface between crystalline Si and the ${\mathrm{SiO}}_{2}$ surface layer. The PL from the crystalline Si core state in large nanocrystals appears in the infrared spectral region. By comparison between surface-oxidized Si nanocrystals and as-prepared porous Si, the size and surface dependence of the coupling between electronic and vibrational excitations are discussed.

Effect of dc electric field on photoluminescence from quantum-confined PbI 2 nanocrystals

The influence of dc electric field is examined on the photoluminescence spectra of various PbI 2 nanocrystals in a poly (vinyl alcohol) matrix, prepared by fast and slow cooling of a hot saturated aqueous PbI 2 solution. The thin flat PbI 2 nanocrystals with a thickness of less than 1 nm exhibit a strong 1D quantum confinement. The application of an electric field of up to 16 kV cm −1 causes a pronounced Stark effect in the differential absorption spectrum and enhancement of excitonic photoluminescence. The last effect results probably from the field-induced detachment of surface-trapped excitons (thermoelectronic ionization or Frenkel effect). While, the larger PbI 2 nanocrystals of size 5 nm did not show an appreciable field-induced effect in the photoluminescence spectra. Such different behaviour between large and small nanocrystals demonstrates the dramatic influence of the surface on the excitonic radiative recombination in semiconductor quantum dots.

Defect Luminescence in Films Containing Ge and GeO2 Nanocrystals

Amorphous SiOx alloys containing Ge or GeO2 nanocrystals are produced by dc-magnetron sputtering and controlled crystallization. The samples are investigated by Raman scattering, transmission electron microscopy, photo luminescence and excitation spectroscopy. Under UV excitation, both types of films luminesce around 3.1 eV, with identical PL line shapes and subnanosecond PL dynamics. The strongest PL intensity is found for the films containing GeO2 crystals and for the largest nanocrystals. These results are a clear indication that although the blue luminescence is without a doubt correlated with the formation of Ge (or GeO2) nanocrystals, it is not produced by the radiative recombination of excitons confined in the nanocrystals. Possible mechanisms for the luminescence are discussed, including defects at the nanocrystal/matrix interface or in the matrix itself.

Adsorbate-Induced Partial Ordering of the Irregular Surface and Subsurface of Crystalline Ice

In our recent studies, evidence was presented that a relaxed ice nanocrystal surface is disordered, in contrast to the nanocrystal interior. In this study it is argued, based on spectroscopic evidence, that disorder extends to several surface layers. Difference spectra between large and small D2O nanocrystals are used to obtain the infrared OD stretch absorption of the disordered subsurface layers. Moreover it is shown that certain select adsorbates at the ice surface induce a significant ordering of the ice subsurface as indicated by the conversion of one-third or more of the subsurface spectrum to that of interior ice. For ice nanocrystals, the new “interior” ice can be greater than 10% of the total amount of ice. Since the ice subsurface apparently consists of approximately three bilayers, this suggests that the influence of these adsorbates on the surface bilayer is reflected in the “ordering” of one or more subsurface bilayers. Examples of adsorbates that have this influence are the bifunctional molecules acetylene and H2S, either of which can act effectively as both proton donor and proton acceptor, while more weakly bonded small molecule adsorbates, such as N2 and CO, do not noticeably influence the subsurface structure. It is suggested that, by engaging in significant H-bonding with the unsaturated surface groups of the top bilayer (dangling-H and dangling-O molecules), the bifunctional adsorbates reverse the restructuring of the outer layer of ice that occurs with an increase of the number of H-bonds of the surface water molecules. This, in turn, reduces the distortion of the ice surface and the displacement of molecules within the ice subsurface layers that accompanies the restructuring. The new data and interpretation give strong support to the view that the equilibrium ice surface has a high degree of structural disorder.

X-ray Photoelectron Spectroscopy of CdSe Nanocrystals with Applications to Studies of the Nanocrystal Surface

We report the use of X-ray photoelectron spectroscopy (XPS) to determine the surface composition of semiconductor nanocrystals. Crystalline, nearly monodisperse CdSe nanocrystals ranging in radius from 9 to 30 A were chemically synthesized and covalently bound to Au and Si surfaces for study. XPS core level peak positions for Cd and Se were in agreement with those of bulk CdSe. We have determined that the majority of Se atoms on the surface are unbonded as prepared and that Cd atoms are bonded to the surface ligand, tri-n-octylphosphine oxide, to the extent that such bonding is sterically allowed. We have determined that the total ligand saturation of the nanocrystal surface varies from 60% in the smaller nanocrystals to 30% in the larger nanocrystals. In addition, we have determined that upon exposure of the nanocrystals to air Se surface sites are oxidized, forming a SeO[sub 2] surface film which causes the nanocrystals to degrade over time. The nanocrystal surface can be modified by dispersing the crystals in pyridine. Nearly all of the P ligands are removed in this case, leaving behind primarily unsaturated Cd and Se surface atoms. In this case, both Cd and Se oxidize upon exposure to air. 35 refs.,more » 17 figs., 1 tab.« less

NMR studies of the surface structure and dynamics of semiconductor nanocrystals

1H NMR studies of thiophenol capping groups on cadmium sulfide nanocrystals demonstrate that the coverage of the capping molecule depends on the size of the nanocrystal. Data are presented which show that as the size of the nanocrystal increases, the coverage of thiophenol decreases. In addition, information about the overall tumbling of the nanocrystal and the motion of the capping groups relative to the surface can be obtained from linewidth studies, indicating that the rotation of the capping groups is hindered in the smaller nanocrystals ( r≈12 Å) and becomes less so in larger nanocrystals ( r≈20 Å). The coverage data are related to the electronic properties of this important class of compounds.

Simulation of Gas Phase Clustering of Nanocrystals in Sputter Discharges

ABSTRACTThe preparation of nanocrystalline particles or clusters (sizes 1–50 nm) is of interest to the study of small systems and for use in sintering or compacting of high purity bulk materials. Recently, a method whereby these crystals can be fabricated using a sputter discharge has been reported. We have developed a computer model to simulate the formation of homogeneous (e.g., Si, Cu, Ti) gas phase clusters in these devices as precursors to larger nanocrystals. The model combines Monte Carlo and drift-diffusion algorithms to simulate the sputtering of atoms from the target, their thermalization in the buffer gas, and gas phase nucleation reactions. Densities of clusters having one to many hundreds of atoms are obtained as a function of position in the discharge. We find that the experimentally observed particle sizes cannot be explained by clustering involving solely neutral reactants due to their short residence times in the plasma. Negatively charged clusters which are trapped in the plasma have correspondingly longer residence times and most likely are responsible for the growth of large particles. Scaling laws for the growth of homogeneous clusters will be presented based on the results of the model.

Semi-conductor impurity and exciton levels in a magnetic field

The paper reviews physical concepts related to the collective dynamics of plasmon excitations in metal nanocrystals with a focus on the photogeneration of energetic carriers. Using quantum linear response theory, we analyze the wave function of a plasmon in nanostructures of different sizes. Energetic carriers are efficiently generated in small nanocrystals due to the non-conservation of momentum of electrons in a confined nanoscale system. On the other hand, large nanocrystals and nanostructures, when driven by light, produce a relatively small number of carriers with large excitation energies. Another important factor is the polarization of the exciting light. Most efficient generation and injection of high-energy carriers can be realized when the optically induced electric current is along the smallest dimension of a nanostructure and also normal to its walls and, for efficient injection, the current should be normal to the collecting barrier. Other important properties and limitations: (1) intra-band transitions are preferable for generation of energetic electrons and dominate the absorption for relatively long wavelengths (approximately >600 nm), (2) inter-band transitions efficiently generate energetic holes and (3) the carrier-generation and absorption spectra can be significantly different. The described physical properties of metal nanocrystals are essential for a variety of potential applications utilizing hot plasmonic electrons including optoelectronic signal processing, photodetection, photocatalysis and solar-energy harvesting.