Bingpo Zhang

Physical approaches to tuning the luminescence color patterns of colloidal quantum dots

Localized surface plasmon resonance (LSPR) and photoactivation (PA) effects are combined for the tuning of fluorescent colors of colloidal CdSe quantum dots (QDs). It is found that LSPR with QD emitters intensely enhances surface state emission, accompanied by a remarkable red-shift of fluorescent colors, while PA treatment with colloidal QDs leads to a distinct enhancement of band-edge emission, accompanied by a peak blue-shift. Furthermore, the LSPR effect on QD emitters can be continuously tuned by the PA process. The combination of the post-synthetic approaches allows feasible realization of multi-color patterns from one batch of QDs and the approaches can also be compatible with other micro-fabrication technologies of QD embossed fluorescent patterns, which undoubtedly provides a way of precisely tuning the colors of light-emitting materials and devices that use colloidal QDs.

Plasmon-enhanced mid-infrared luminescence from polar and lattice-structure-mismatched CdTe/PbTe single heterojunctions

We propose by exploring CdTe/PbTe single heterojunctions (SHs) that the abnormal enhancement of mid-infrared light emission from CdTe/PbTe heterostructures is due to coupling with surface plasmons. It is discovered that the observed intense mid-infrared luminescence in the SHs comes from the inherent polar interface character and coupling of surface plasmons localized at the metallic CdTe/PbTe interface to light emitted from the narrow gap PbTe. The finding offers an approach to manipulate mid-infrared light and to fabricate improved mid-infrared optoelectronic devices.

Band alignment determination of ZnO/PbSe heterostructure interfaces by synchrotron radiation photoelectron spectroscopy

Band offsets at ZnO/PbSe heterostructure interfaces are determined by synchrotron radiation photoelectron spectroscopy. A type-I band alignment with the valence band offset (VBO) ?EV?=?0.73?eV and conduction band offset (CBO) ?EC?=?2.36?eV is concluded for ZnO/PbSe film heterojunctions. The approach is extended to ZnO/PbSe nanocrystal (NC) heterojunctions, which reveals that band alignment can be adjusted via varying the dot size and a type-II alignment is formed when the dot size is ?5?nm. The small conduction band offset of the ZnO/PbSe film heterojunction and the tunable band alignment of the ZnO/PbSe NCs heterojunction with different crystal sizes shall benefit the design and fabrication of improved optoelectronic devices.

Quantum Oscillations in a Two-Dimensional Electron Gas at the Rocksalt/Zincblende Interface of PbTe/CdTe (111) Heterostructures

Quantum oscillations are observed in the 2DEG system at the interface of novel heterostructures, PbTe/CdTe (111), with nearly identical lattice parameters (a(PbTe) = 0.6462 nm, a(CdTe) = 0.648 nm) but very different lattice structures (PbTe: rock salt, CdTe: zinc blende). The 2DEG formation mechanism, a mismatch in the bonding configurations of the valence electrons at the interface, is uniquely different from the other known 2DEG systems. The aberration-corrected scanning transmission electron microscope (AC-STEM) characterization indicates an abrupt interface without cation interdiffusion due to a large miscibility gap between the two constituent materials. Electronic transport measurements under magnetic field up to 60 T, with the observation of Landau level filling factor ν = 1, unambiguously reveal a π Berry phase, suggesting the Dirac Fermion nature of the 2DEG at the heterostructure interface, and the PbTe/CdTe heterostructure being a new candidate for 2D topological crystalline insulators.

Designable Luminescence with Quantum Dot–Silver Plasmon Coupler

We explore a strongly interacting QDs/Ag plasmonic coupling structure that enables multiple approaches to manipulate light emission from QDs. Group II-VI semiconductor QDs with unique surface states (SSs) impressively modify the plasmonic character of the contiguous Ag nanostructures whereby the localized plasmons (LPs) in the Ag nanostructures can effectively extract the non-radiative SSs of the QDs to radiatively emit via SS-LP resonance. The SS-LP coupling is demonstrated to be readily tunable through surface-state engineering both during QD synthesis and in the post-synthesis stage. The combination of surface-state engineering and band-tailoring engineering allows us to precisely control the luminescence color of the QDs and enables the realization of white-light emission with single-size QDs. Being a versatile metal, the Ag in our optical device functions in multiple ways: as a support for the LPs, for optical reflection, and for electrical conduction. Two application examples of the QDs/Ag plasmon coupler for optical devices are given, an Ag microcavity + plasmon-coupling structure and a new QD light-emitting diode. The new QDs/Ag plasmon coupler opens exciting possibilities in developing novel light sources and biomarker detectors.

Resonant nature of intrinsic defect energy levels in PbTe revealed by infrared photoreflectance spectroscopy

Step-scan Fourier-transform infrared photoreflectance and modulated photoluminescence spectroscopy were used to characterize the optical transitions of the epitaxial PbTe thin film grown by molecular beam epitaxy on BaF2 (111) substrate in the vicinity of energy gap of lead telluride at 77 K. It is found that the intrinsic defect energy levels in the electronic structure are of resonant nature. The Te-vacancy energy level is located above the conduction band minimum by 29.1 meV. Another defect (VX) energy level situated below valance band maximum by 18.1 meV is also revealed. Whether it is associated with the Pb vacancy is still not clear. It might also be related to the misfit dislocations stemming from the lattice mismatch between PbTe and BaF2 substrate. The experimental results support the theory prediction (N. J. Parada and G. W. Pratt, Jr., Phys. Rev. Lett. 22, 180 (1969), N. J. Parada, Phys. Rev. B 3, 2042 (1971)) and are consistent with the reported Hall experimental results (G. Bauer, H. Burkhard, H. Heinrich, and A. Lopez-Otero, J. Appl. Phys. 47, 1721 (1976)).

Twisted ZB?CdTe/RS?PbTe (111) heterojunction as a metastable interface structure

The heterostructures of (zinc-blende)-CdTe/(rock-salt)-PbTe are typically found to have their common cubic axes aligned to each other, as in the case of PbTe quantum dots embedded in a CdTe matrix. In this work, we perform both theoretical and experimental studies on the CdTe/PbTe heterostructure in a different geometry: a planar CdTe/PbTe (111) heterostructure. We simulate the epitaxial growth of CdTe (PbTe) on the (111) PbTe (CdTe) substrate, using a density-functional theory. A twisted CdTe/PbTe (111) interface structure has been predicted in the layer-by-layer epitaxial growth on the (111) substrate, in contrast to the non-twisted CdTe/PbTe (111) interface reported in the literature. This predicted structure has been confirmed experimentally in the heterostructure grown by molecular beam epitaxy, using a high-resolution transmission electron microscope. The twisted interface has a lower binding energy than the non- twisted one, indicating that the twisted structure is a metastable phase formed in

Enhancement of light emission from nanostructured In 2 O 3 via surface plasmons

We report the construction of In(2)O(3)/Ag/In(2)O(3) sandwich nanostructures and realization of effective coupling with surface plasmon (SP) modes. An enhancement of photoluminescence as large as 278-fold is achieved for the new nanostructures, while only eightfold is obtained from bilayer structures. The advancement of the nanostructures is that both the frequency of incidence photons and the in-plane wavevector of the excited SP modes along each side of the sandwiched nanometer metal layer are identical, thus the momenta mismatch between two SP modes which inevitably occurs in commonly used metal/dielectric bilayer structures is no longer a problem. The fulfillment of the cross coupling and resonance conditions of the two SP modes leads to the tremendous amplification of light emission. Such sandwich nanostructures can be readily extended to other dielectric/metal/dielectric nanomaterial combinations and identified as technologically useful for SP mediated light emitting devices.

A dry method to synthesize dendritic Ag2Se nanostructures utilizing CdSe quantum dots and Ag thin films

Dendritic Ag2Se nanostructures are synthesized in a dry environment by UV irradiating the hybrids composed of CdSe quantum dots (QDs) and silver (Ag). UV irradiation on CdSe QDs induces a photooxidation effect on the QD surface and leads to the formation of SeO2 components. Then SeO2 reacts with the Ag atoms in either Ag film or QD layer to produce the Ag2Se. The growth mechanism of Ag2Se dendrites on solid Ag films is explored and explained by a diffusion limited aggregation model in which the QD layer provides enough freedom for Ag2Se motion. Since the oxidation of the CdSe QDs is the critical step for the Ag2Se dendrites formation this dry chemical interaction between QDs and Ag film can be applied in the study of the QD surface chemical properties. With this dry synthesis method, the Ag2Se dendrites can also be facilely formed at the designed area on Ag substrates.

Phonon blocking by two dimensional electron gas in polar CdTe/PbTe heterojunctions

Narrow-gap lead telluride crystal is an important thermoelectric and mid-infrared material in which phonon functionality is a critical issue to be explored. In this Letter, efficient phonon blockage by forming a polar CdTe/PbTe heterojunction is explicitly observed by Raman scattering. The unique phonon screening effect can be interpreted by recent discovery of high-density two dimensional electrons at the polar CdTe/PbTe(111) interface which paves a way for design and fabrication of thermoelectric devices.