CuCl Nanocrystals Research Articles

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.

CuCl nanocrystals in alkali-halide matrices under hydrostatic pressure

CuCl nanocrystals in crystalline alkali-halide matrices have been investigated under hydrostatic pressures up to 18 GPa. The pressures of structural phase transitions in CuCl have been determined both for different nanocrystal sizes and for different matrices (NaCl, LiCl, KCl). For CuCl nanocrystals in NaCl an increase of the transition pressure with decreasing nanocrystal size is observed, which is explained by the increasing importance of surface pressure for small nanocrystals. We found higher transition pressures for the LiCl matrix than for the NaCl matrix. The reason for this is that the pressure which acts on the nanocrystal differs from the external pressure. A simple elastic model describes the effective pressure transmitted from the matrix to the nanocrystal. With CuCl nanocrystals embedded in KCl we have studied the behavior of nanocrystals during a phase transition of the matrix. Additionally we have determined the pressure coefficients of the exciton energies of the CuCl nanocrystals, which depend on the elastic properties of the matrix.

Size dependence of confined acoustic phonons in CuCl nanocrystals

Persistent spectral hole burning spectroscopy was used to study the size dependence of the confined acoustic phonons in CuCl nanocrystals embedded in silicate glass, NaCl, and KCl. It is found that the energies of the confined acoustic phonons in the nanocrystals in glass and KCl are almost the same, but twice larger than those in NaCl, which were obtained from the Stokes-side acoustic phonon holes with the same excitation energies. The confined acoustic phonons in the nanocrystals in glass and KCl can be well explained in terms of the lowest-frequency vibrational modes calculated on a sphere model with a free boundary condition. However, the energies of the confined acoustic phonons in the nanocrystals in NaCl are lower than the frequencies of the lowest-frequency vibrational modes predicted by a cube model with a free boundary condition. This observation shows that the energies of the confined acoustic phonons in CuCl nanocrystals depend on the size, the shape, and the boundary condition of the nanocrystals.

Two-photon excitation spectra of confined excitons in CuCl nanocrystals

We investigated two-photon excitation spectra of confined excitons of CuCl nanocrystals embedded in a NaCl matrix. The spectra exhibit a sharp increase that is attributed to transitions into higher Coulombic states $(\mathrm{nP})$ of the confined excitons, and its position shifts to a higher energy side with decreasing crystal size. We also discuss the difference in peak positions between one- and two-photon absorption spectra in the presence of a broad inhomogeneous width due to the size distribution.

Quantum beats of confined exciton-LO phonon complexes in CuCl nanocrystals

We have investigated coherent transient phenomena of confined excitons in CuCl nanocrystals with mean radii R in the range 3.4--23 nm, embedded in glass by means of femtosecond degenerate four-wave mixing (DFWM). Oscillating behaviors have been observed in the spectrally resolved DFWM signals. In large nano- crystals with $R&gt;6.5$ nm, quantum beats between the confined exciton sublevels have been observed, while for $R&lt;6.5$ nm we have observed quantum beats between the confined exciton and LO-phonon-assisted transitions. The LO-phonon energy extracted from the oscillation period is reduced with decreasing crystallite size, which is interpreted in terms of LO-phonon renormalization in the presence of an exciton.

Excited-state absorption of excitons confined in spherical quantum dots

The excited-state photoabsorption spectrum by an electron and a hole confined in spherical quantum dots is calculated. As the dot size is reduced, the calculated absorption peak shifts to the high-energy side, and splits into two peaks in the limit of strong confinement. This splitting represents the changeover of the quantum size effect of the zero-dimensional excitons. Our theoretical result is in agreement with experimental data for the transient infrared absorption spectra under size-selective excitation observed for CuCl nanocrystals embedded in NaCl crystals.

Theory of multi-exciton states in semiconductor nanocrystals

In this article, the fundamental physics of multi-exciton states in semiconductor nano-crystals is reviewed focusing on the mesoscopic enhancement of the excitonic radiative decay rate and the excitonic optical nonlinearity and the mechanism of their saturation with increase of the nanocrystal size. In the case of the radiative decay rate the thermal excitation of excited exciton states having small oscillator strength within the homogeneous linewidth of the exciton ground state is essential in determining the saturation behavior. The weakly correlated exciton pair states are found to cause a cancellation effect in the third-order nonlinear optical susceptibility at the exciton resonance, providing the first consistent understanding of the experimentally observed saturation of the mesoscopic enhancement of the excitonic optical nonlinearity. The presence of the weakly correlated exciton pair states is confirmed convincingly from the good correspondence between theory and experiments on the induced absorption spectra from the exciton state in CuCl nanocrystals. Furthermore, ultrafast relaxation processes of biexcitons are discussed in conjunction with the observed very fast rise of the biexciton gain in nanocrystals. In prospect of future progress in research, the theoretical formulation to calculate the triexciton states as one of the multi-exciton states beyond the biexciton is presented for the first time including the electron-hole exchange interaction.

Size effects in the exciton energy and first-order phase transitions in CuCl nanocrystals in glass

The absorption spectra and the melting and crystallization kinetics of CuCl nanocrystals in glass are investigated in the range of particle radii 1–30 nm. Three discontinuities are found on the curves representing the size dependence of the melting point Tm(R) and the crystallization point Tc(R). As the particle radius gradually decreases from 30 nm in the range R⩽12.4 nm there is a sudden 60° drop in the temperature Tc in connection with the radius of the critical CuCl nucleus in the melt. A 30° drop in Tm is observed at R=2.1 nm, and a second drop of 16° in the temperature Tc is observed for CuCl particles of radius 1.8 nm. The last two drops are associated with changes in the equilibrium shape of the nanoparticles. In the range of smaller particles, R⩽1.34 nm the Tc(R) curve is observed to merge with the Tm(R) curve, owing to the disappearance of the work of formation of the crystal surface during crystallization of the melt as a result of the zero surface tension of CuCl particles of radii commensurate with the thickness of the effective surface layer. An increase in the size shift of the exciton energy is observed in this same range of CuCl particle radii (1–1.8 nm). The size dependence of the melting and crystallization temperatures of the nanoparticles is attributed to variation of the free energy in the surface layer of a particle.

Optical determination of the size distribution of CuCl nanocrystals in NaCl

We demonstrate the model-free determination of the size-distribution function f( R) of CuCl nanocrystals in NaCl by analyzing the absorption spectrum of the Z 3 exciton of CuCl. It is proved that the maximum of the exciton absorption coincides with the mean radius R̄. The number of nanocrystals with small sizes is much larger than predicted for Ostwald ripening. Thus, in addition to ripening processes as described by the popular Lifshitz–Slyozov–Wagner growth theory other mechanisms occur during cluster growth.

LO Phonon Renormalization in Optically Excited CuCl Nanocrystals

We have revealed sharp structures in the absorption spectrum of CuCl nanocrystals using persistent hole-burning spectroscopy and observed sidebands due to phonon-assisted absorption. The LO phonon frequency in the excited state of the nanocrystal is reduced by about 10% from the free value due to mixing with the exciton. When the mixing becomes resonant, the phonon sideband anticrosses with the excited exciton state. We present a theory of the phonon renormalization in nanocrystals and obtain results in excellent agreement with the experiment.

Relaxation processes of photoexcited carriers in CuCl nanocrystals embedded in NaCl

In a CuCl nanocrystal embedded in a NaCl single crystal, we observed a time response of exciton and biexciton luminescence intensities under strong pulsed light excitation by using the up-conversion method at 2 and 77 K. It was found that the rise of the biexciton luminescence is delayed compared to that of the exciton luminescence at 77 K. This is assumed to be because the biexciton is not directly created from electron-hole $(e\ensuremath{-}h)$ pairs but through exciton formation. On the basis of the cascade relaxation model, the formation time of an $e\ensuremath{-}h$ pair to an exciton is estimated to be 1.6 ps. We also observed a time response of luminescence intensities of the bound exciton, free biexciton, and bound biexciton at 2 K. These time profiles are interpreted by the cascade relaxation process. The formation time of the $e\ensuremath{-}h$ pair is estimated to be 1.3 ps, and the trapping lifetimes of the exciton to a surface impurity center and of the $e\ensuremath{-}h$ pair to the bound exciton are estimated to be 1.4 and 1.8 ps, respectively.

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.

Quantum-size effects on the exciton excited states of CuCl nanocrystals

We have investigated infrared transient absorption at 77 K, which appears under size-selective excitation of I S excitons in CuCl nanocrystals embedded in Nad matrices. The transient absorption is mainly ascribed to the optical transition between the 1S to 2P exciton states in CuCl nanocrystals. The peak energy of the transient absorption band increases by about 200 meV with the decrease in the crystal size from more than 10 nm down to 1.4 nm in effective radii, while the width of the band increases by more than 100 meV. These facts indicate that the quantum size effect on the exciton excited state is much deviated from the exciton confinement regime and the hydrogen-like P exciton components are widely distributed among the various excited confinement states split by the quantum size effect.

Time-resolved study of two-photon excitation of CuCl nanocrystals

We investigated the ps time-resolved emission spectra of CuCl nanocrystals under resonant two-photon excitation. The results clarified the interchanging existence of the resonant luminescence and the second-harmonic scattering, depending on the excitation energy,

Correlation betweenCu+-ion instability and persistent spectral hole-burning phenomena of CuCl nanocrystals

We have studied persistent spectral hole-burning (PSHB) phenomena of CuCl nanocrystals embedded in a NaCl crystal. We found correlations between the PSHB phenomena and the photoluminescence spectral changes of excitons in CuCl nanocrystals and of ${\mathrm{Cu}}^{+}$ dimers in a NaCl crystal. The spectral changes can be triggered by photoinduced ${\mathrm{Cu}}^{+}$-ion displacements in CuCl nanocrystals and in a NaCl crystal.

CuCl Nanocrystals in Alkali Halide Matrices: Determination of Growth Parameters by Exciton Spectroscopy

At low temperatures the absorption spectra of CuCl-doped NaCl single crystals show the Z3 and Z1, 2 excitons of CuCl if CuCl exists in the crystalline state. A procedure is demonstrated for dividing the total oscillator strength observed between 3.0 and 5.5 eV into two parts, one is associated with the crystalline state of CuCl and the other with single copper ions. The excitons show blue shifts as a consequence of confinement effects that permit the determination of the average radius R of the CuCl nanocrystals. The transition from initial growth processes to the regime of Ostwald-ripening is demonstrated for growth temperatures between room temperature and 600 K.

Exciton–LO-phonon interaction in CuCl spherical quantum dots studied by resonant hyper-Raman spectroscopy

Resonant hyper-Raman-scattering spectroscopy was applied for studies of the exciton--LO-phonon interaction in spherical CuCl nanocrystals of different sizes embedded in a glass matrix. Both the LO and 2LO phonon bands have shown a prominent resonance with the lowest energy $1S$ confined exciton state. The ratio of the integral intensities of the LO and 2LO bands \ensuremath{\rho} was found to increase with increase of the incident photon energy, or decrease of the nanocrystal radius. The Huang-Rhys factor $S$ has been calculated as a function of the nanocrystal size from the experimentally measured values of \ensuremath{\rho} on the offset harmonic-oscillator model of electron-vibrational coupling. It has been found that $S$ increases monotonically from 0.22 to 0.7, with the nanocrystal radius decreasing from 3.6 to 1.6 nm.

Temperature dependent optical resonances of quantum confined CuCl nanocrystals in borosilicate glasses

The optical density change of the CuCl-doped borosilicate glasses (2 wt% CuCl +52.5SiO2−40B2O3−7.5Na2O(wt%)) heat treated at 550 °C for 0–10 hrs was measured with temperature at spectral vicinity of the CuCl resonant absorption and the effect of size and distribution of CuCl precipitate particles on the optical spectra was investigated. The optical absorption spectrum obtained during heating and cooling cycle showed a typical hysteresis loop because of the melting and solidification of the CuCl particles embedded in the glass matrix. The shape of the optical absorption curve measured at 375 nm with temperature upon melting of CuCl particles was dependent on the size and the distribution of the CuCl particles. While the average size of the CuCl particles increased with heat treatment time, the size difference of the precipitated CuCl particles was found to be decreased with time before 1 h and then increased. The optical absorption spectra of the CuCl doped glass was also predicted theoretically and the predicted spectra curve was found to be in good agreement with experimental data. It is proposed that if the semiconducting particles embedded in an optically transparent matrix and the melting point of semiconductor is lower than that of the matrix material, the average size and size distribution of the particles can be estimated by the optical absorption measurements with temperature.

Reconsideration of relaxation processes of excitons in CuCl nanocrystals

We report temporal changes of luminescence and induced absorption of excitons in nanometer-size semiconductor crystals (nanocrystals) of CuCl embedded in NaCl or in glass, and discuss relaxation mechanisms of the excitons. In CuCl nanocrystals in NaCl, luminescence-decay or -rise is observed at “free”- or “localized”-state luminescence, respectively. This indicates nonradiative relaxation from the “free” exciton to the “localized” state. In CuCl nanocrystals in glass, the temporal changes of the induced absorption have a longer decay component, in addition to a fast decay component which agrees with the luminescence decay. This result suggests exciton decomposition and existence of an electron remainder in the nanocrystals.

Two-photon excitation and luminescence of confined excitons in CuCl nanocrystals

We investigated two-photon excitation and subsequent emission due to confined excitons of CuCl nanocrystals embedded in NaCl matrices. The resonant emission spectra indicate the existence of ‘two-photon resonant luminescence’ and ‘secondharmonic scattering’. The excitation spectra exhibit sharp increase which may be assigned to the transitions into higher Coulombic states ( nP) of the confined excitons.