Raman spectroscopy and structure of crystalline gallium phosphide nanowires

On the origins of anomalous elastic moduli and failure strains of GaP nanowires

Previous reports suggest that Raman peaks in uniaxially loaded nanowires with diamond cubic and zinc blende crystal structures shift at rates that are significantly different from bulk specimens. We have investigated the first order Raman scattering from individual, free-standing, [111] oriented GaP nanowires ranging from 75 to 180 nm in diameter at uniaxial tensile stresses up to 5 GPa. All of the phonon modes were shifted to frequencies lower than previously reported for bulk GaP, and significant splitting of the degenerate transverse optical mode was observed. A general analysis method using single and double Lorentzian fits of the Raman peaks is presented and used to report more accurate values of the phonon deformation potentials (PDPs) that relate uniaxial strains to Raman peak shifts in GaP. A new set of PDPs determined from the nanowires revealed that the they have elastic moduli and failure strains that are consistent with bulk GaP. The analysis method eliminated the anomalous, inconsistent deformation behavior commonly reported in Raman-based strain measurements of nanowires, and can be extended to other materials systems with degenerate phonons.

Recent developments and future directions in the growth of nanostructures by van der Waals epitaxy

Here we review the characteristics of "van der Waals epitaxy" (vdWE) as an alternative epitaxy mechanism that has been demonstrated as a viable method for circumventing the lattice matching requirements for epitaxial growth. Particular focus is given on the application of vdWE for nonplanar nanostructures. We highlight our works on the vdWE growth of nanowire arrays, tripods, and tetrapods from various semiconductors (ZnO, ZnTe, CdS, CdSe, CdSxSe1-x, CdTe, and PbS) on muscovite mica substrates, irrespective of the ensuing lattice mismatch. We then address the controllability of the synthesis and the growth mechanism of ZnO nanowires from catalyst-free vdWE in vapor transport growth. As exemplified herein with optical characterizations of ZnO and CdSe nanowires, we show that samples from vdWE may possess properties that are as excellent as those from conventional epitaxy. With our works, we aim to advocate vdWE as a prospective universal growth strategy for nonplanar epitaxial nanostructures.

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.

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.

Raman spectroscopy of few-quintuple layer topological insulator Bi2Se3 nanoplatelets

We report on Raman spectroscopy of few quintuple layer topological insulator bismuth selenide (Bi2Se3) nanoplatelets (NPs), synthesized by a polyol method. The as-grown NPs exhibit excellent crystalline quality, hexagonal or truncated trigonal morphology, and uniformly flat surfaces down to a few quintuple layers. Both Stokes and anti-Stokes Raman spectroscopy for the first time resolve all four optical phonon modes from individual NPs down to 4 nm, where the out-of-plane vibrational A(1g)(1) mode shows a few wavenumbers red shift as the thickness decreases below ~15 nm. This thickness-dependent red shift is tentatively explained by a phonon softening due to the decreasing of the effective restoring force arising from a decrease of the van der Waals forces between adjacent layers. Quantitatively, we found that the 2D phonon confinement model proposed by Faucet and Campbell cannot explain the red shift values and the line shape of the A(1g)(1) mode, which can be described better by a Breit–Wigner–Fano resonance line shape. Considerable broadening (~17 cm(–1) for six quintuple layers) especially for the in-plane vibrational mode E(g)(2) is identified, suggesting that the layer-to-layer stacking affects the intralayer bonding. Therefore, a significant reduction in the phonon lifetime of the in-plane vibrational modes is probably due to an enhanced electron–phonon coupling in the few quintuple layer regime.

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.

Structural, electronic, optical and vibrational properties of nanoscale carbons and nanowires: a colloquial review

This review addresses the field of nanoscience as viewed through the lens of the scientific career of Peter Eklund, thus with a special focus on nanocarbons and nanowires. Peter brought to his research an intense focus, imagination, tenacity, breadth and ingenuity rarely seen in modern science. His goal was to capture the essential physics of natural phenomena. This attitude also guides our writing: we focus on basic principles, without sacrificing accuracy, while hoping to convey an enthusiasm for the science commensurate with Peter's. The term 'colloquial review' is intended to capture this style of presentation. The diverse phenomena of condensed matter physics involve electrons, phonons and the structures within which excitations reside. The 'nano' regime presents particularly interesting and challenging science. Finite size effects play a key role, exemplified by the discrete electronic and phonon spectra of C(60) and other fullerenes. The beauty of such molecules (as well as nanotubes and graphene) is reflected by the theoretical principles that govern their behavior. As to the challenge, 'nano' requires special care in materials preparation and treatment, since the surface-to-volume ratio is so high; they also often present difficulties of acquiring an experimental signal, since the samples can be quite small. All of the atoms participate in the various phenomena, without any genuinely 'bulk' properties. Peter was a master of overcoming such challenges. The primary activity of Eklund's research was to measure and understand the vibrations of atoms in carbon materials. Raman spectroscopy was very dear to Peter. He published several papers on the theory of phonons (Eklund et al 1995a Carbon 33 959-72, Eklund et al 1995b Thin Solid Films 257 211-32, Eklund et al 1992 J. Phys. Chem. Solids 53 1391-413, Dresselhaus and Eklund 2000 Adv. Phys. 49 705-814) and many more papers on measuring phonons (Pimenta et al 1998b Phys. Rev. B 58 16016-9, Rao et al 1997a Nature 338 257-9, Rao et al 1997b Phys. Rev. B 55 4766-73, Rao et al 1997c Science 275 187-91, Rao et al 1998 Thin Solid Films 331 141-7). His careful sample treatment and detailed Raman analysis contributed greatly to the elucidation of photochemical polymerization of solid C(60) (Rao et al 1993b Science 259 955-7). He developed Raman spectroscopy as a standard tool for gauging the diameter of a single-walled carbon nanotube (Bandow et al 1998 Phys. Rev. Lett. 80 3779-82), distinguishing metallic versus semiconducting single-walled carbon nanotubes, (Pimenta et al 1998a J. Mater. Res. 13 2396-404) and measuring the number of graphene layers in a peeled flake of graphite (Gupta et al 2006 Nano Lett. 6 2667-73). For these and other ground breaking contributions to carbon science he received the Graffin Lecture award from the American Carbon Society in 2005, and the Japan Carbon Prize in 2008. As a material, graphite has come full circle. The 1970s renaissance in the science of graphite intercalation compounds paved the way for a later explosion in nanocarbon research by illuminating many beautiful fundamental phenomena, subsequently rediscovered in other forms of nanocarbon. In 1985, Smalley, Kroto, Curl, Heath and O'Brien discovered carbon cage molecules called fullerenes in the soot ablated from a rotating graphite target (Kroto et al 1985 Nature 318 162-3). At that time, Peter's research was focused mainly on the oxide-based high-temperature superconductors. He switched to fullerene research soon after the discovery that an electric arc can prepare fullerenes in bulk quantities (Haufler et al 1990 J. Phys. Chem. 94 8634-6). Later fullerene research spawned nanotubes, and nanotubes spawned a newly exploding research effort on single-layer graphene. Graphene has hence evolved from an oversimplified model of graphite (Wallace 1947 Phys. Rev. 71 622-34) to a new member of the nanocarbon family exhibiting extraordinary electronic properties. Eklund's career spans this 35-year odyssey.

Polarized Rayleigh back-scattering from individual semiconductor nanowires

A complete understanding of the interaction between electromagnetic radiation and semiconductor nanowires (NWs) is required in order to further develop a new generation of opto-electronic and photonic devices based on these nanosystems. The reduced dimensionality and high aspect ratio of nanofilaments can induce strong polarization dependence of the light absorption, emission and scattering, leading in some cases to the observation of optical antenna effects. In this work we present the first systematic study of polarized Rayleigh back-scattering from individual crystalline semiconductor NWs with known crystalline structure, orientation and diameters. To explain our experimental Rayleigh polar patterns, we propose a simple theory that relies on a secondary calculation of the volume-averaged internal electromagnetic fields inside the NW. These results revealed that the internal and emitted field can be enhanced depending on the polarization with respect to the NW axis; we also show that this effect strongly depends on the NW diameter.

Cavity-enhanced stimulated raman scattering from short GaP nanowires

We report the first observation of stimulated Raman scattering (SRS) from semiconductor nanowires (SNWs). Using continuous wave (CW) excitation (514.5 nm), very strong nonlinear SRS was observed in backscattering from short segments of crystalline GaP NWs with diameter d = 210 nm when the wire length L < 1.1 microm. The SRS intensity was found to increase dramatically with decreasing L down to 270 nm. The effective threshold pump power P(T) needed to observe the SRS is quite small. Indeed, we observe values of P(T) 3 orders of magnitude smaller than reported for bulk crystals of strongly nonlinear LiIO(3) and Ba(NO(3))(2) and 2 orders of magnitude smaller than that reported recently for SRS from "top-down" fabricated silicon microwaveguides. P(T) was observed to decrease linearly with decreasing L. Our data are discussed in terms of theoretical results developed for dielectric cavities. The quality factors Q for the short GaP cylindrical cavities studied here are estimated to be approximately 15,000. Our results suggest that SNWs may find applications as Raman lasers.

Optical antenna effect in semiconducting nanowires

We report on investigations of the interaction of light with nanoscale antennae made from crystalline GaP nanowires (NWs). Using Raman scattering, we have observed strong optical antenna effects which we identify with internal standing wave photon modes of the wire. The antenna effects were probed in individual NWs whose diameters are in the range 40 < d < 300 nm. The data and our calculations show that the nature of the backscattered light is critically dependent on the interplay between a photon confinement effect and bulk Raman scattering. At small diameter, d < 65 nm, the NWs are found to act like a nearly perfect dipole antenna and the bulk Raman selection rules are masked leading to a polarized scattering intensity function I R approximately cos4 theta. Underscoring the importance of this work is the realization that a fundamental understanding of the "optical antenna effect" in semiconducting NWs is essential to the analysis of all electro-optic effects in small diameter filaments.

Coherent twinning phenomena: towards twinning superlattices in III-V semiconducting nanowires

We report evidence in GaP and InP nanowires for a coherent modulation of the structure along the wire axis. By using electron diffraction, we have observed an additional series of diffraction peaks consistent with a quasiperiodic placement of twinning boundaries along the wire. This observation is indeed unexpected, as the vapor-liquid-solid growth conditions used to produce the nanowires were not modulated. The averaged repeat distance of the structure, i.e., the distance between twin boundaries, has been found to depend on the temperature gradient imposed in the growth zone. Future control of the twinning superlattice period should allow significant design possibilities for electronic, thermoelectric, thermal and electro-optic applications of semiconducting nanowires.

Confinement effects on optical phonons in polar tetrapod nanocrystals detected by resonant inelastic light scattering

We investigated CdTe nanocrystal tetrapods of different sizes by resonant inelastic light scattering at room temperature and under cryogenic conditions. We observe a strongly resonant behavior of the phonon scattering with the excitonic structure of the tetrapods. Under resonant conditions we detect a set of phonon modes that can be understood as confined longitudinal-optical phonons, surface-optical phonons, and transverse-optical phonons in a nanowire picture.