2014 New's Items

Facile Electron-Beam Lithography Technique for Irregular and Fragile Substrates

J.Y. Chang
Q. Zhou
A. Zettl
2014
A facile technique is presented which enables high-resolution electron beam lithography on irregularly-shaped, non-planar or fragile substrates such as the edges of a silicon chip, thin and narrow suspended beams and bridges, or small cylindrical wires. The method involves a spin-free dry-transfer of pre-formed uniform-thickness polymethyl methacrylate, followed by conventional electron beam writing, metal deposition, and lift-off. High-resolution patterning is demonstrated for challenging target substrates. The technique should find broad application in micro- and nano-technology research...

Three-Dimensional Spirals of Atomic Layered MoS2

L. Zhang
K. Liu
A. B. Wong
J. Kim
X. Hong
C. Liu
T. Cao
S. G. Louie
F. Wang
P. Yang
2014

Atomically thin two-dimensional (2D) layered materials, including graphene, boron nitride, and transition metal dichalcogenides (TMDs), can exhibit novel phenomena distinct from their bulk counterparts and hold great promise for novel electronic and optoelectronic applications. Controlled growth of such 2D materials with different thickness, composition, and symmetry are of central importance to realize their potential. In particular, the ability to control the symmetry of TMD layers is highly desirable because breaking the inversion symmetry can lead to intriguing valley physics,...

Coherent and Diffusive Timescales for Exciton Dissociation in Bulk Heterojunction Photovoltaic Cells

K. B. Whaley
A. Kocherzhenko
A. Nitzan
2014

We study the dynamics of charge separation in bulk heterojunction organic photovoltaic systems in light of recent experimental observations that this process is characterized by multiple time scales in the range of 10 fs to 100 ps. Coherent evolution of the excitonic state has been suggested to dominate the early stages of the charge separation process and diffusion of localized excitons to be dominant at longer times. Both of these processes obviously depend on the system morphology, in particular on the grain sizes of the donor and acceptor phases. Here we analyze these mechanisms...

Symmetry Breaking in Tetrahedral Chiral Plasmonic Nanoparticle Assemblies

V. E. Ferry
J. M. Smith
A. P. Alivisatos
2014

Self-assembled plasmonic structures combine the specificity and tunability of chemical synthesis with collective plasmonic properties. Here we systematically explore the effects of symmetry breaking on the chiroptical response of an assembly of plasmonic nanoparticles using simulation. The design is based on a tetrahedral nanoparticle frame with two different types of nanoparticles, where chirality is induced by targeted stimuli that change the distance along one edge of the assembly. We show that the intensity, spectral position, and handedness of the CD response are tunable with...

Synthesis and Characterization of Highly Crystalline Graphene Aerogels

M.A. Worsley
T.T. Pham
A. Yan
S.J. Shin
J.R.I. Lee
M. Bagge-Hansen
W. Mickelson
A. Zettl
2014

Aerogels are used in a broad range of scientific and industrial applications due to their large surface areas, ultrafine pore sizes, and extremely low densities. Recently, a large number of reports have described graphene aerogels based on the reduction of graphene oxide (GO). Though these GO-based aerogels represent a considerable advance relative to traditional carbon aerogels, they remain significantly inferior to individual graphene sheets due to their poor crystallinity. Here, we report a straightforward method to synthesize highly crystalline GO-based graphene aerogels ...

Tunable Electrical Conductivity in Oriented Thin Films of Tetrathiafulvalene-Based Covalent Organic Framework

S.-L. Cai
Y.-B. Zhang
A. B. Pun
B. He
J. Yang
F. M. Toma
I. D. Sharp
O. M. Yaghi
J. Fan
S.-R. Zheng
W.-G. Zhang
Y. Liu
2014

Despite the high charge-carrier mobility in covalent organic frameworks (COFs), the low intrinsic conductivity and poor solution processability still impose a great challenge for their applications in flexible electronics. We report the growth of oriented thin films of a tetrathiafulvalene-based COF (TTF-COF) and its tunable doping. The porous structure of the crystalline TTF-COF thin film allows the diffusion of dopants such as I2 and tetracyanoquinodimethane (TCNQ) for redox reactions, while the closely packed 2D grid sheets facilitate the...

Multijunction Solar Cell Efficiencies: Effect of Spectral Window, Optical Environment and Radiative Coupling

C. N. Eisler
Z. R. Abrams
M. T. Sheldon
X. Zhang
H. A. Atwater
2014

Solar cell efficiency is maximized through multijunction architectures that minimize carrier thermalization and increase absorption. Previous proposals suggest that the maximum efficiency for a finite number of subcells is achieved for designs that optimize for light trapping over radiative coupling. We instead show that structures with radiative coupling and back reflectors for light trapping, e.g. spectrum-splitting cells, can achieve higher conversion efficiencies. We model a compatible geometry, the polyhedral specular reflector. We analyze and...

Ligand Dissociation Mediated Charge Transfer Observed at Colloidal W18O49 Nanoparticle Interfaces

D. C. Grauer
A. P. Alivisatos
2014

Understanding charge transfer dynamics through the ligand shell of colloidal nanoparticles has been an important pursuit in solar energy conversion. While charge transport through ligand shells of nanoparticle films has been studied intensely in static dry and electrochemical systems, its influence on charge transfer kinetics in dispersed colloidal systems has received relatively less attention. This work reports the oxidation of amine passivated tungsten oxide nanoparticles by an organically soluble tris-(1,10-phenanthroline) ironIII derivative. By...

PbS Nanoparticles Capped with Tetrathiafulvalenetetracarboxylate: Utilizing Energy Level Alignment for Efficient Carrier Transport

M. Scheele
D. Hanifi
D. Zherebetskyy
S. T. Chourou
S. Axnanda
B. J. Rancatore
K. Thorkelsson
T. Xu
Z. Liu
L.-W. Wang
Y. Liu
A. P. Alivisatos
2014

We fabricate a field-effect transistor by covalently functionalizing PbS nanoparticles with tetrathiafulvalenetetracarboxylate. Following experimental results from cyclic voltammetry and ambient-pressure X-ray photoelectron spectroscopy, we postulate a near-resonant alignment of the PbS 1Sh state and the organic HOMO, which is confirmed by atomistic calculations. Considering the large width of interparticle spacing, we observe an abnormally high field-effect hole mobility, which we attribute to the postulated resonance. In contrast to nanoparticle devices...

Polymer-Encapsulated Metallic Nanoparticles as a Bridge Between Homogeneous and Heterogeneous Catalysis

E. Gross
F. D. Toste
G. A. Somorjai
2014

Continuous efforts in catalysis research have been devoted towards the development of heterogeneous catalysts that can activate reactions which are catalyzed by homogeneous catalysts. Replacing homogeneous catalysts with their heterogeneous counterparts will enhance the sustainability of the catalytic system, providing a highly recyclable, scalable and efficient setup. Throughout this review we demonstrate that small (<2 nm), metallic nanoclusters can catalyze a wide range of π-bond activation reactions that were previously activated by homogeneous catalysts. The small size of the...