Xiaobin Niu

Wedding Cake Growth Mechanism in One-Dimensional and Two-Dimensional Nanostructure Evolution.

The kinetic processes and atomistic mechanisms in nanostructure growth are of fundamental interest to nanomaterial syntheses with precisely controlled morphology and functionality. By programming deposition conditions at time domain, we observed the wedding cake growth mechanism in the formation of 1D and 2D ZnO nanostructures. Within a narrow growth window, the surfaces of the 1D and 2D structures were covered with a unique concentric terrace feature. This mechanism was further validated by comparing the characteristic growth rates to the screw dislocation-driven model. An interesting 1D to 2D morphology transition was also found during the wedding cake growth, when the adatoms overcome the Ehrlich-Schwoebel (ES) barrier along the edge of the top crystal facet triggered by lowering the supersaturation. The wedding cake model might be a general growth mechanism for flat-tipped nanowires that do not possess any dislocations. This study enriches our understanding on the fundamental kinetics of nanostructured crystal growth and provides a transformative strategy to achieve rational design and control of nanoscale geometry.

Effects of Plasmonic Metal Core -Dielectric Shell Nanoparticles on the Broadband Light Absorption Enhancement in Thin Film Solar Cells

To guide the design of plasmonic solar cells, theoretical investigation of core (metal)-shell (dielectric) nanoparticles for light absorption enhancement in thin film Si solar cells is performed. In contrast to the reported simulations and experimental results that rear-located surface plasmon on bare metallic nanoparticles is preferred, the core-shell nanoparticles demonstrate better performance when surface plasmon is located in front of a solar cell. This has been attributed to the enhanced forward scattering with vanishing backward scattering preserved over a wide spectral range in core-shell nanoparticles. This work provides a concept to achieve enhanced forward scattering with weakened backward scattering in plasmonic thin film solar cells.

Phase separation in strained epitaxial InGaN islands

Phase separation (PS) produces InN composition gradients in InGaN islands, which may be important for light emitting diodes, solar cells, and lasers. Thus, the control of PS is critical, and the kinetic growth process, which is suggested to be important for controlling PS in Stranski-Krastanov islands, becomes a key factor in producing materials for optoelectronic devices. We present atomistic-strain-model Monte Carlo simulations for PS in strained epitaxial InGaN islands. Our simulations illustrate how the PS in InGaN islands depends on the kinetic growth mode and subsurface diffusion, and thus suggest ideas for controlling the microstructure of alloy islands formed during epitaxial growth.

A level set simulation for ordering of quantum dots via cleaved-edge overgrowth

Cleaved-edge overgrowth (CEO) is a promising technique to obtain ordered arrays of quantum dots, where the size and position of the dots can be controlled very well. We present level set simulations for CEO. Our simulations illustrate how the quality of the CEO technique depends on the potential energy surface (PES) for adatom diffusion, and thus suggest how variations of the PES can potentially improve the uniformity of quantum dot arrays.

Thermodynamic Self-Limiting Growth of Heteroepitaxial Islands Induced by Nonlinear Elastic Effect

We investigate nonlinear elastic effect (NLEF) on the growth of heteroepitaxial islands, a topic of both scientific and technological significance for their applications as quantum dots. We show that the NLEF induces a thermodynamic self-limiting growth mechanism that hinders the strain relaxation of coherent island beyond a maximum size, which is in contrast to indefinite strain relaxation with increasing island size in the linear elastic regime. This self-limiting growth effect shows a strong dependence on the island facet angle, which applies also to islands inside pits patterned in a substrate surface with an additional dependence on the pit inclination angle. Consequently, primary islands nucleate and grow first in the pits and then secondary islands nucleate at the rim around the pits after the primary islands reach the self-limited maximum size. Our theory sheds new lights on understanding the heteroepitaxial island growth and explains a number of past and recent experimental observations.

Epitaxial growth and thermal-conductivity limit of single-crystalline Bi2Se3/In2Se3 superlattices on mica

Thermal transport in superlattices is governed by various phonon-scattering processes. For extracting the phonon-scattering contribution of hetero-interfaces in chalcogenide superlattices, single-crystalline Bi2Se3/In2Se3 (BS/IS) superlattices with minimized defects are prepared on fluorophlogopite mica by molecular beam epitaxy. The cross-plane heat-conducting properties of the BS/IS superlattices are demonstrated to depend precisely on the period thicknesses and constituents of the superlattices, where a minimum in the thermal conductivity indicates a crossover from particle-like to wave-like phonon transport in the superlattices. The thermal-conductivity minimum of the BS/IS superlattices is nearly one order of magnitude lower than that of intrinsic BS film.

Ion-Beam-Directed Self-Ordering of Ga Nanodroplets on GaAs Surfaces

Ordered nanodroplet arrays and aligned nanodroplet chains are fabricated using ion-beam-directed self-organization. The morphological evolution of nanodroplets formed on GaAs (100) substrates under ion beam bombardment is characterized by scanning electron microscopy and atomic force microscopy. Ordered Ga nanodroplets are self-assembled under ion beam bombardment at off-normal incidence angles. The uniformity, size, and density of Ga nanodroplets can be tuned by the incident angles of ion beam. The ion beam current also plays a critical role in the self-ordering of Ga nanodroplets, and it is found that the droplets exhibit a similar droplet size but higher density and better uniformity with increasing the ion beam current. In addition, more complex arrangements of nanodroplets are achieved via in situ patterning and ion-beam-directed migration of Ga atoms. Particularly, compared to the destructive formation of nanodroplets through direct ion beam bombardment, the controllable assembly of nanodroplets on intact surfaces can be used as templates for fabrication of ordered semiconductor nanostructures by droplet epitaxy.

Monolithic integration of metastable α-In2Se3 thin film on H-passivated Si(1 1 1) for photovoltaic applications

The metastable α-In2Se3 thin film is epitaxially integrated on H-passivated Si (1 1 1) substrates to build a novel heterojunction solar cell by molecular beam epitaxy. The growth of In2Se3 on H–Si(1 1 1) at low temperature initiates as an amorphous layer then followed by re-crystalline of α phase film. Electronic transport properties of α-In2Se3/p-Si heterostructure are studied. Analysis of the temperature dependence of thermionic emission in reverse bias indicates a barrier height of ~0.21 eV at the α-In2Se3/p-Si interface. A photovoltaic conversion efficiency of 2% is measured for the heterojunction with optimized In2Se3 film thickness.

Anchoring and space-confinement effects to form ultrafine Ru nanoclusters for efficient hydrogen generation

Developing highly efficient, durable, and low-cost catalysts for the hydrogen evolution reaction (HER) is an eternal pursuit for scientists to replace Pt-based catalysts. The fundamental ways to boost the performance of electrocatalysts are improving the intrinsic activity of each active site and increasing the number of exposed active sites. Herein, we report a novel approach to synthesize ultrafine Ru nanocluster catalysts embedded in a nitrogen-doped carbon framework. To illustrate this strategy, Ru atoms are immobilized in tetra-aminephthalocyanine (RuPc-NH2) by Ru–N4 bonds and subsequently confined by graphene oxide (GO) nanosheets in space. After pyrolysis treatment, ultrafine Ru nanoclusters (average size 1.03 ± 0.23 nm) stabilized by an N-doped carbon framework with a 23.7 wt% Ru content (Ru@NG) are prepared. Importantly, Ru@NG shows excellent HER performance with small overpotentials (20.3 mV in 1.0 M KOH and 42.7 mV in 0.5 M H2SO4) at 10 mA cm−2, a high active site number (3.54 × 10−3 mol g−1) and robust durability both in alkaline and acidic environments. Therefore, this work provides a promising substitute for expensive Pt-based catalysts.

Efficiency enhancement in inverted planar perovskite solar cells by synergetic effect of sulfated graphene oxide (sGO) and PEDOT:PSS as hole transporting layer

Inverted planar perovskite solar cells (PSCs) exhibiting a high power conversion efficiency (PCE) have mainly been demonstrated by using poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the hole transport layer (HTL). As an alternative to the PEDOT:PSS, graphene oxide (GO) is also employed as a HTL in PSCs with decent PCEs. However, the strong acidity and hygroscopicity of PEDOT:PSS and insulting property of GO were the major factors for hindering the fabrication of high-performance PSCs. Here, we demonstrated sulfated graphene oxide (sGO) as a HTL replacing the conventionally used GO and PEDOT:PSS in PSCs, but pristine sGO as simple HTL cannot improve photovoltaic performance of PSCs with a maximum efficiency of 9.9%. Hence, we report a simple solution route for preparing a sGO–PEDOT:PSS composite HTL by combining solution-processable sGO with commercialized PEDOT:PSS solution. The PSC fabricated with 1 : 1 sGO–PEDOT:PSS HTL shows a dramatically enhanced PCE of 13.9%, versus 11.5% for PSC with pristine PEDOT:PSS HTL. This promising strategy could be a critical step toward the ideal HTL design for the advancement of practical perovskite solar cells.