A. G. Milekhin

Resonant Raman studies of compositional and size dispersion of CdS1-xSex nanocrystals in a glass matrix

Resonant Raman scattering spectra of glass-embedded CdS1−x Sex nanocrystals are measured and complemented with TEM and optical absorption as well as photoluminescence data. The selectivity of the resonant Raman process not only fo rt he size, but also for the composition of nanocrystals within the ensemble, is directly observed in the dependence of phonon band frequency, linewidth and shape on the excitation wavelength.

Optical vibrational modes in (Cd, Pb, Zn)S quantum dots embedded in Langmuir–Blodgett matrices

Abstract Structures with CdS, PbS, ZnS quantum dots (QDs) produced using the Langmuir–Blodgett technique were investigated by infrared (IR), Raman and ultra-violet–visible (UV–Vis) spectroscopies. The QDs size derived from the analysis of UV–Vis spectra and high-resolution transmission electron microscopy images amounts to 2–6 nm. The IR and Raman spectra reveal longitudinal optical phonons localised in QDs and surface vibrational modes. The frequency positions of the surface optical vibrational modes are adequately described taking into account confinement of fundamental optical phonons in the QDs.

Size-Dependent Non-FRET Photoluminescence Quenching in Nanocomposites Based on Semiconductor Quantum Dots CdSe/ZnS and Functionalized Porphyrin Ligands

We review recent experimental work to utilize the size dependence of the luminescence quenching of colloidal semiconductor quantum dots induced by functionalized porphyrin molecules attached to the surface to describe a photoluminescence (PL) quenching process which is different from usual models of charge transfer (CT) or Foerster resonant energy transfer (FRET). Steady-state and picosecond time-resolved measurements were carried out for nanocomposites based on colloidal CdSe/ZnS and CdSe quantum dots (QDs) of various sizes and surfacely attached tetra-mesopyridyl-substituted porphyrin molecules (“Quantum Dot-Porphyrin” nanocomposites), in toluene at 295 K. It was found that the major part of the observed strong quenching of QD PL in “QD-Porphyrin” nanocomposites can neither be assigned to FRET nor to photoinduced charge transfer between the QD and the chromophore. This PL quenching depends on QD size and shell and is stronger for smaller quantum dots: QD PL quenching rate constants 𝑘𝑞 scale inversely with the QD diameter. Based on the comparison of experimental data and quantum mechanical calculations, it has been concluded that QD PL quenching in “QD-Porphyrin” nanocomposites can be understood in terms of a tunneling of the electron (of the excited electron-hole pair) followed by a (self-) localization of the electron or formation of trap states. The major contribution to PL quenching is found to be proportional to the calculated quantum-confined exciton wave function at the QD surface. Our findings highlight that single functionalized molecules can be considered as one of the probes for the complex interface physics and dynamics of colloidal semiconductor QD.

Raman study of self-assembled SiGe nanoislands grown at low temperatures

We report on Raman scattering measurements on Si-capped Ge quantum structures grown by molecular beam epitaxy on Si(001) at low temperatures. We find a double band structure in the Ge–Ge frequency range for nanoislands grown at substrate temperatures ranging in the interval 300–500 °C. Complementary information has been obtained from performing Raman scattering experiments on annealed samples. The results are interpreted in terms of a model that considers quantum structures (hut clusters) composed of a strained Ge core and a more relaxed SiGe shell.

Combination of surface- and interference-enhanced Raman scattering by CuS nanocrystals on nanopatterned Au structures

Summary We present the results of a Raman study of optical phonons in CuS nanocrystals (NCs) with a low areal density fabricated through the Langmuir–Blodgett technology on nanopatterned Au nanocluster arrays using a combination of surface- and interference-enhanced Raman scattering (SERS and IERS, respectively). Micro-Raman spectra of one monolayer of CuS NCs deposited on a bare Si substrate reveal only features corresponding to crystalline Si. However, a new relatively strong peak occurs in the Raman spectrum of CuS NCs on Au nanocluster arrays at 474 cm−1. This feature is related to the optical phonon mode in CuS NCs and manifests the SERS effect. For CuS NCs deposited on a SiO2 layer this phonon mode is also observed due to the IERS effect. Its intensity changes periodically with increasing SiO2 layer thickness for different laser excitation lines and is enhanced by a factor of about 30. CuS NCs formed on Au nanocluster arrays fabricated on IERS substrates combine the advantages of SERS and IERS and demonstrate stronger SERS enhancement allowing for the observation of Raman signals from CuS NCs with an ultra-low areal density.

Size-selective Raman scattering in self-assembled Ge/Si quantum dot superlattices

Self-organized Ge quantum dot (QD) superlattices having properties of two- and zero-dimensional structures were investigated by Raman spectroscopy. Longitudinal optical (LO) Ge phonons, longitudinal (L) Ge-Si phonons and folded longitudinal acoustic (LA) phonons superimposed on a strong continuous emission were studied under resonant conditions. The measured phonon frequencies of folded LA phonons up to 15th order are in good agreement with those calculated using the Rytov model applied usually to planar superlattices. The low-frequency continuous emission can be explained in terms of a breakdown of crystal momentum conservation for resonant Raman processes involving acoustic phonons. A frequency enlargement of the continuous emission band and a downward shift of the LO Ge phonons with increasing excitation energy (2.54-2.71 eV) are attributed to electron and phonon size confinement in the small Ge QDs resonantly contributing to the scattering process. The size of the QDs involved in the process is estimated from analysis of the frequency position of their optical phonons.

Vibrational spectra of quantum dots formed by Langmuir–Blodgett technique

Phonon spectra of CdS, ZnS, PbS, CuS, Ag2S, and ZnO quantum dots formed by using the Langmuir–Blodgett technology are investigated by Raman and infrared spectroscopies. The Raman spectra of structures show peaks corresponding to the scattering by longitudinal and transverse optical (LO and TO) phonons localized in quantum dots that confirm the formation of nanocrystals. In addition to TO and LO phonon modes, the modes observed in the IR spectra between the frequency positions of LO and TO phonons are attributed to the surface optical phonons in quantum dots.

Characterization of low-temperature wafer bonding by infrared spectroscopy

We present the results of an infrared (IR) spectroscopic investigation of interfaces between two hydrophilic Si wafers bonded at low temperature. Multiple internal transmission IR spectra were recorded of the bonds, with different chemical pretreatments of Si surfaces employed before bonding. The analysis of IR spectra shows that the number of O–H and H–Si–Ox species at the interface depends strongly on the chemical pretreatment type, which determines the bonding energy. The annealing procedure used in the bonding process leads to dissociation of water molecules, oxidation of silicon at the interfaces, and diffusion of hydrogen into silicon oxide layer formed at the interface. The difference in bonding processes is discussed.

A study of the vertical transport of electrons in (GaAs)n(AlAs)m superlattices by Fourier transform infrared spectroscopy

The longitudinal optical vibrational modes have been studied in (GaAs)n(AlAs)m superlattices by means of FTIR spectroscopy. In the undoped samples, confined LO phonons have been observed; the dispersion of LO phonons obtained by FTIR spectroscopy was in good agreement with the Raman data. In the doped (GaAs)n(AlAs)m superlattices, coupling of confined vibrational LO modes with superlattice plasmons has been found. Analysis of the experimental results showed that the frequency of superlattice plasmons in the superlattices studied is mostly determined by the population of the miniband formed by the Gamma -like conduction band states. The fitting of the calculated reflection spectra to the experimental spectra allowed us to measure the population of the minibands, the effective mass and the vertical mobility of electrons.

Temperature-dependent Raman investigation of rolled up InGaAs/GaAs microtubes

Large arrays of multifunctional rolled-up semiconductors can be mass-produced with precisely controlled size and composition, making them of great technological interest for micro- and nano-scale device fabrication. The microtube behavior at different temperatures is a key factor towards further engineering their functionality, as well as for characterizing strain, defects, and temperature-dependent properties of the structures. For this purpose, we probe optical phonons of GaAs/InGaAs rolled-up microtubes using Raman spectroscopy on defect-rich (faulty) and defect-free microtubes. The microtubes are fabricated by selectively etching an AlAs sacrificial layer in order to release the strained InGaAs/GaAs bilayer, all grown by molecular beam epitaxy. Pristine microtubes show homogeneity of the GaAs and InGaAs peak positions and intensities along the tube, which indicates a defect-free rolling up process, while for a cone-like microtube, a downward shift of the GaAs LO phonon peak along the cone is observed. Formation of other type of defects, including partially unfolded microtubes, can also be related to a high Raman intensity of the TO phonon in GaAs. We argue that the appearance of the TO phonon mode is a consequence of further relaxation of the selection rules due to the defects on the tubes, which makes this phonon useful for failure detection/prediction in such rolled up systems. In order to systematically characterize the temperature stability of the rolled up microtubes, Raman spectra were acquired as a function of sample temperature up to 300°C. The reversibility of the changes in the Raman spectra of the tubes within this temperature range is demonstrated.