ABSTRACT
Narrow bandgap (<0.5 eV) colloidal semiconductor nanocrystals (e.g. mercury chalcogenides) provide practical platforms for next generation short wave infrared, mid wave infrared and long wave infrared optoelectronic devices. Until now, most of the efforts in the field of infrared active nanocrystals have been taken on synthesizing nanocrystals, determining quantum states and building different geometries for optoelectronic devices. However, studies on interface trap states in the devices made from these narrow band gap nanocrystals are mostly unexplored. Herein, we investigate the defects or traps in these nanocrystals-embedded devices, which will be critical for improving their optoelectronic performance. In this article, we fabricate HgTe nanocrystals/TiO2 based photovoltaic devices and used capacitance-voltage (C-V) and deep level transient spectroscopy (DLTS) to investigate and obtain quantitative information on deep level trap states. Interestingly, frequency dependent C-V measurements show two peaks in the capacitance at lower frequency (<40 kHz), which is attributed to the presence of trap states. However, at high frequency the presence of a weak hump-like structure almost at the center of above two peaks validate the role of interface traps. DLTS studies show that traps at the interface of HgTe nanocrystals/TiO2 acts as recombination centers having activation energies of 0.27, 0.4 and 0.45 eV with corresponding trap densities of 1.4 [Formula: see text], 1.[Formula: see text] and 1.[Formula: see text] and estimated capture cross-sections of 6.3 [Formula: see text], 7.5 [Formula: see text] and 3.7 [Formula: see text], respectively. In this work, DLTS has revealed the existence of interface trap states and the frequency dependent capacitance measurements corroborate the effect of charge storage on the heterostructures built from these nanocrystals that helps in the development of futuristic devices.
ABSTRACT
We report the synthesis of nanocrystals with an optical feature in the THz range. To do so, we develop a new synthetic procedure for the growth of HgTe, HgSe, and HgS nanocrystals, with strong size tunability from 5 to 200 nm. This is used to tune the absorption of the nanocrystals all over the infrared range up to terahertz (from 2 to 65 µm for absorption peak and even 200 µm for cutoff wavelength). The interest for this procedure is not limited to large sizes since for small objects we demonstrate low aggregation and good shape control (i.e., spherical object) while using nonexpansive and simple mercury halogenide precursors. By integrating these nanocrystals into an electrolyte-gated transistor, we evidence a change of carrier density from p-doped to n-doped as the confinement is vanishing.
ABSTRACT
Mercury chalcogenide nanocrystals and especially HgTe appear as an interesting platform for the design of low cost mid-infrared (mid-IR) detectors. Nevertheless, their electronic structure and transport properties remain poorly understood, and some critical aspects such as the carrier relaxation dynamics at the band edge have been pushed under the rug. Some of the previous reports on dynamics are setup-limited, and all of them have been obtained using photon energy far above the band edge. These observations raise two main questions: (i) what are the carrier dynamics at the band edge and (ii) should we expect some additional effect (multiexciton generation (MEG)) as such narrow band gap materials are excited far above the band edge? To answer these questions, we developed a high-bandwidth setup that allows us to understand and compare the carrier dynamics resonantly pumped at the band edge in the mid-IR and far above the band edge. We demonstrate that fast (>50 MHz) photoresponse can be obtained even in the mid-IR and that MEG is occurring in HgTe nanocrystal arrays with a threshold around 3 times the band edge energy. Furthermore, the photoresponse can be effectively tuned in magnitude and sign using a phototransistor configuration.
ABSTRACT
Naturally formed CdTe/CdS core/shell quantum dot (QD) structures in the presence of surface stabilizing agents have been synthesized by a hydrothermal method. Size and temperature dependent photoluminescence (PL) spectra have been investigated to understand the exciton-phonon interaction, and radiative and nonradiative relaxation of carriers in these QDs. The PL of these aqueous CdTe QDs (3.0-4.8 nm) has been studied in the temperature range 15-300 K. The strength of the exciton-LO-phonon coupling, as reflected in the Huang-Rhys parameter 'S' is found to increase from 1.13 to 1.51 with the QD size varying from 4.8 to 3.0 nm. The PL linewidth (FWHM) increases with increase in temperature and is found to have a maximum in the case of QDs of 3.0 nm in size, where the exciton-acoustic phonon coupling coefficient is enhanced to 51 µeV K(-1), compared to the bulk value of 0.72 µeV K(-1). To understand the nonradiative processes, which affect the relaxation of carriers, the integrated PL intensity is observed as a function of temperature. The integrated PL intensity remains constant until 50 K for relatively large QDs (3.9-4.8 nm) beyond which a thermally activated process takes over. Below 150 K, a small activation energy, 45-19 meV, is found to be responsible for the quenching of the PL. Above 150 K, the thermal escape from the dot assisted by scattering with multiple longitudinal optical (LO) phonons is the main mechanism for the fast quenching of the PL. Besides this high temperature quenching, interestingly for relatively smaller size QDs (3.4-3.0 nm), the PL intensity enhances as the temperature increases up to 90-130 K, which is attributed to the emission of carriers from interface/trap states having an activation energy in the range of 6-13 meV.
