Muhammad Iqbal Bakti Utama

Polarity Assignment in ZnTe, GaAs, ZnO, and GaN-AlN Nanowires from Direct Dumbbell Analysis

Aberration corrected scanning transmission electron microscopy (STEM) with high angle annular dark field (HAADF) imaging and the newly developed annular bright field (ABF) imaging are used to define a new guideline for the polarity determination of semiconductor nanowires (NWs) from binary compounds in two extreme cases: (i) when the dumbbell is formed with atoms of similar mass (GaAs) and (ii) in the case where one of the atoms is extremely light (N or O: ZnO and GaN/AlN). The theoretical fundaments of these procedures allow us to overcome the main challenge in the identification of dumbbell polarity. It resides in the separation and identification of the constituent atoms in the dumbbells. The proposed experimental via opens new routes for the fine characterization of nanostructures, e.g., in electronic and optoelectronic fields, where the polarity is crucial for the understanding of their physical properties (optical and electronic) as well as their growth mechanisms.

Vertically aligned cadmium chalcogenide nanowire arrays on muscovite mica: a demonstration of epitaxial growth strategy.

We report a strategy for achieving epitaxial, vertically aligned cadmium chalcogenide (CdS, CdSe, and CdTe) nanowire arrays utilizing van der Waals epitaxy with (001) muscovite mica substrate. The nanowires, grown from a vapor transport process, exhibited diameter uniformity throughout their length, sharp interface to the substrate, and positive correlation between diameter and length with preferential growth direction of [0001] for the monocrystalline wurtzite CdS and CdSe nanowires, but of [111] for zinc blende CdTe nanowires, which also featured abundant twinning boundaries. Self-catalytic vapor-liquid-solid mechanism with hydrogen-assisted thermal evaporation is proposed to intepret the observations. Optical absorption from the as-grown CdSe nanowire arrays on mica at 10 K revealed intense first-order exciton absorption and its longitudinal optical phonon replica. A small Stokes shift (∼1.3 meV) was identified, suggesting the high quality of the nanowires. This study demonstrated the generality of van der Waals epitaxy for the growth of nanowire arrays and their potential applications in optical and energy related devices.

Incommensurate van der Waals epitaxy of nanowire arrays: a case study with ZnO on muscovite mica substrates.

The requirement of lattice matching between a material and its substrate for the growth of defect-free heteroepitaxial crystals can be circumvented with van der Waals epitaxy (vdWE). However, the utilization and characteristics of vdWE in nonlamellar/nonplanar nanoarchitectures are still not very well-documented. Here we establish the characteristics of vdWE in nanoarchitectures using a case study of ZnO nanowire (NW) array on muscovite mica substrate without any buffer/seed layer. With extensive characterizations involving electron microscopy, diffractometry, and the related analyses, we conclude that the NWs grown via vdWE exhibit an incommensurate epitaxy. The incommensurate vdWE allows a nearly complete lattice relaxation at the NW-substrate heterointerface without any defects, thus explaining the unnecessity of lattice matching for well-crystallized epitaxial NWs on muscovite mica. We then determine the polarity of the NW via a direct visualization of Zn-O dumbbells using the annular bright field scanning transmission electron miscroscopy (ABF-STEM) in order to identify which atoms are at the base of the NWs and responsible for the van der Waals interactions. The information from the ABF-STEM is then used to construct the proper atomic arrangement at the heterointerface with a 3D atomic modeling to corroborate the characteristics of the incommensurate vdWE. Our findings suggest that the vdWE might be extended for a wider varieties of compounds and epitaxial nanoarchitectures to serve as a universal epitaxy strategy.

Composition-Tunable Vertically Aligned CdSxSe1-x Nanowire Arrays via van der Waals Epitaxy: Investigation of Optical Properties and Photocatalytic Behavior

Utilization of semiconductor ternary alloys – including the pseudobinary instances when the mixture is composed of two binary semiconductors with a common element – offers the benefi t of continuous tunability of bandgap to suit the requirement of specifi c photonic devices of interest. The bandgap of a pseudobinary alloy can also be tuned conveniently simply by adjusting the stoichiometry of its binary constituents, which otherwise have discrete and limited bandgap values. However, the accompanying changes of the cell parameters of the material with respect to the composition create a diffi culty in achieving wellcrystalized growth of bulk alloys with complete composition tunability epitaxially, despite the strong importance of epitaxy for applications demanding a reliable and reproducible device fabrication. At some compositions, expansion or contraction of the in-plane lattice of the alloy will worsen the lattice mismatch with the substrate, which may then promote nucleation of mismatch-related dislocations and defects that are deleterious to the optical and electrical properties of the alloy. [ 1 ] To resolve the issue of lattice mismatch and the ensuing defects, growth of epitaxial alloy in vertically aligned nanowire array geometry has been proposed [ 2 ] due to the possibility of nanowires to relieve strain in the lateral direction owing to their fi nite diameter. [ 3 ] In addi

Excitonic Properties and Near‐Infrared Coherent Random Lasing in Vertically Aligned CdSe Nanowires

One-dimensional (1D) semiconductor nanowires (NWs) have drawn considerable research attention over the past few decades because of their unique properties and potential applications in nanoelectronics, photonics, luminescent materials, lasing materials, and biological and medical sensing. [ 1–3 ] Impressive progress has been demonstrated in highly effi cient light sources (nanolasers), waveguides, fi eld-effect transistors and photodetectors based on group IV elements (Si and Ge), [ 4 ] III–V compound semiconductors (GaN and GaAs), [ 2 , 5 ] and semiconducting oxides (ZnO, SnO 2 and In 2 O 3 ). [ 6 , 7 ] Owing to its direct bandgap (ca. 1.74 eV at room temperature), good absorption ability, and excellent photosensitivity, [ 8 , 9 ] CdSe is recognized as a promising light-harvesting material to be applied in optoelectronics. Especially, the fundamental emission of CdSe falls in the near-infrared (NIR) region, and biosensors operating in this region can avoid interference from biological media such as tissue autofl uorescence and scattering light, and thereby facilitate relatively interference-free sensing. [ 10 ]

Epitaxial II–VI Tripod Nanocrystals: A Generalization of van der Waals Epitaxy for Nonplanar Polytypic Nanoarchitectures

We report for the first time the synthesis of nonplanar epitaxial tripod nanocrystals of II-VI compounds (ZnO, ZnS, ZnSe, ZnTe, CdS, CdSe, and CdTe) on muscovite mica substrate. With CdS as a case study, we conclude via Raman spectroscopy and electron microscopy studies that the tripods, which are found to be polytypic, followed a seeded growth mechanism. The epitaxy, manifested by the in-plane alignment of the legs of the tripods within a substrate, is attributed to the van der Waals interaction between the tripod bases and the mica surface, instead of to the covalent chemical bond which would require lattice matching between the epilayer and the substrate. The results demonstrated herein could have widespread immediate implications, including the potential of van der Waals epitaxy to be applicable in producing ordered arrays of more complex nanoarchitectures from various classes of compounds toward a broad range of technological applications.

Heteroepitaxial Decoration of Ag Nanoparticles on Si Nanowires: A Case Study on Raman Scattering and Mapping

Metallic nanoparticle-decorated silicon nanowires showed considerable promise in a wide range of applications such as photocatalytic conversion, surface-enhanced Raman scattering, and surface plasmonics. However there is still insufficient amount of Raman scattering in Si nanowires with such decoration. Here we report the heteroepitaxial growth of Ag nanoparticles on Si nanowires by a surface reduction mechanism. The as-grown Ag nanoparticles exhibited highly single crystallinity with a most probable diameter of 25 nm. Raman scattering spectroscopy showed a new sideband feature at 495 cm(-1) below the first order Si transverse optical Raman peak due to HF etching. This new feature sustained after sequential surface treatments and rapid thermal annealing, therefore was attributed to polycrystalline defect at subsurface, which was confirmed by high-resolution transmission electron microscopy observations. Correlated atomic force microscopy and Raman mapping demonstrated that single Ag nanoparticle decoration significantly enhanced Raman signal of Si nanowire by a factor of 7, suggesting that it would be a promising approach to probe phonon confinement and radial breathing mode in individual nanowires down to sub-10 nm regime.

Highly Enhanced Exciton Recombination Rate by Strong Electron− Phonon Coupling in Single ZnTe Nanobelt

Electron-phonon coupling plays a key role in a variety of elemental excitations and their interactions in semiconductor nanostructures. Here we demonstrate that the relaxation rate of free excitons in a single ZnTe nanobelt (NB) is considerably enhanced via a nonthermalized hot-exciton emission process as a result of an ultrastrong electron-phonon coupling. Using time-resolved photoluminescence (PL) spectroscopy and resonant Raman spectroscopy (RRS), we present a comprehensive study on the identification and the dynamics of free/bound exciton recombination and the electron-phonon interactions in crystalline ZnTe NBs. Up to tenth-order longitudinal optical (LO) phonons are observed in Raman spectroscopy, indicating an ultrastrong electron-phonon coupling strength. Temperature-dependent PL and RRS spectra suggest that electron-phonon coupling is mainly contributed from Light hole (LH) free excitons. With the presence of hot-exciton emission, two time constants (∼80 and ∼18 ps) are found in photoluminescence decay curves, which are much faster than those in many typical semiconductor nanostructures. Finally we prove that under high excitation power amplified spontaneous emission (ASE) originating from the electron-hole plasma occurs, thereby opening another radiative decay channel with an ultrashort lifetime of few picoseconds.

Size and surface effects on transient photoconductivity in CdS nanobelts probed by time-resolved terahertz spectroscopy

Ultrafast optical-pump terahertz probe spectroscopy was performed over a graduated size distribution of CdS nanobelts to investigate the size and surface effects on the transient photoconductivity. It was found that the nanobelt size has a profound influence on the carrier localization and photoconductivity dynamics, brought about by the carrier trapping at surface defects. The strong carrier localization in the nanobelt is ascribed to the internal surface boundaries arising from the surface depletion layer. The increased thickness of surface depletion layer due to a continuous trapping of photocarriers at surface defects results in more pronounced carrier localization after photoexcitation.

II–VI compound semiconductor nanowires: Optical properties and nanophotonics

Abstract This chapter discusses the optical properties, in particular the luminescent properties, of II–VI compound semiconductor nanowires and their applications in nanophotonics. We will first review the various synthesis methods and especially the most recent advances in the van der Waals epitaxy in growing nonplanar vertically aligned nanowires. Then, we focus on the steady-state and time-resolved dynamics of the excitons in typical nanowires. Finally, various waveguides and resonator structures based on II–VI semiconductor nanowires, as well as different types of nanowire lasers, will be discussed.