6 research outputs found
Atomic-Scale Interfacial Band Mapping across Vertically Phased-Separated Polymer/Fullerene Hybrid Solar Cells
Using
cross-sectional scanning tunneling microscope (XSTM) with
samples cleaved in situ in an ultrahigh vacuum chamber, this study
demonstrates the direct visualization of high-resolution interfacial
band mapping images across the film thickness in an optimized bulk
heterojunction polymer solar cell consisting of nanoscale phase segregated
blends of polyÂ(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric
acid methyl ester (PCBM). We were able to achieve the direct observation
of the interfacial band alignments at the donor (P3HT)-acceptor (PCBM)
interfaces and at the interfaces between the photoactive P3HT:PCBM
blends and the polyÂ(3,4-ethylenedioxythiophene) polyÂ(styrenesulfonate)
(PEDOT:PSS) anode modification layer with an atomic-scale spatial
resolution. The unique advantage of using XSTM to characterize polymer/fullerene
bulk heterojunction solar cells allows us to explore simultaneously
the quantitative link between the vertical morphologies and their
corresponding local electronic properties. This provides an atomic
insight of interfacial band alignments between the two opposite electrodes,
which will be crucial for improving the efficiencies of the charge
generation, transport, and collection and the corresponding device
performance of polymer solar cells
Dopant Diffusion and Activation in Silicon Nanowires Fabricated by ex Situ Doping: A Correlative Study via Atom-Probe Tomography and Scanning Tunneling Spectroscopy
Dopants play a critical
role in modulating the electric properties of semiconducting materials,
ranging from bulk to nanoscale semiconductors, nanowires, and quantum
dots. The application of traditional doping methods developed for
bulk materials involves additional considerations for nanoscale semiconductors
because of the influence of surfaces and stochastic fluctuations,
which may become significant at the nanometer-scale level. Monolayer
doping is an ex situ doping method that permits the post growth doping
of nanowires. Herein, using atom-probe tomography (APT) with subnanometer
spatial resolution and atomic-ppm detection limit, we study the distributions
of boron and phosphorus in ex situ doped silicon nanowires with accurate
control. A highly phosphorus doped outer region and a uniformly boron
doped interior are observed, which are not predicted by criteria based
on bulk silicon. These phenomena are explained by fast interfacial
diffusion of phosphorus and enhanced bulk diffusion of boron, respectively.
The APT results are compared with scanning tunneling spectroscopy
data, which yields information concerning the electrically active
dopants. Overall, comparing the information obtained by the two methods
permits us to evaluate the diffusivities of each different dopant
type at the nanowire oxide, interface, and core regions. The combined
data sets permit us to evaluate the electrical activation and compensation
of the dopants in different regions of the nanowires and understand
the details that lead to the sharp p–i–n junctions formed
across the nanowire for the ex situ doping process
Atomically Resolved Electronic States and Correlated Magnetic Order at Termination Engineered Complex Oxide Heterointerfaces
We
map electronic states, band gaps, and interface-bound charges
at termination-engineered BiFeO<sub>3</sub>/La<sub>0.7</sub>Sr<sub>0.3</sub>MnO<sub>3</sub> interfaces using atomically resolved cross-sectional
scanning tunneling microscopy. We identify a delicate interplay of
different correlated physical effects and relate these to the ferroelectric
and magnetic interface properties tuned by engineering the atomic
layer stacking sequence at the interfaces. This study highlights the
importance of a direct atomically resolved access to electronic interface
states for understanding the intriguing interface properties in complex
oxides
Spatially Resolved Imaging on Photocarrier Generations and Band Alignments at Perovskite/PbI<sub>2</sub> Heterointerfaces of Perovskite Solar Cells by Light-Modulated Scanning Tunneling Microscopy
The
presence of the PbI<sub>2</sub> passivation layers at perovskite crystal
grains has been found to considerably affect the charge carrier transport
behaviors and device performance of perovskite solar cells. This work
demonstrates the application of a novel light-modulated scanning tunneling
microscopy (LM-STM) technique to reveal the interfacial electronic
structures at the heterointerfaces between CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> perovskite crystals and PbI<sub>2</sub> passivation
layers of individual perovskite grains under light illumination. Most
importantly, this technique enabled the first observation of spatially
resolved mapping images of photoinduced interfacial band bending of
valence bands and conduction bands and the photogenerated electron
and hole carriers at the heterointerfaces of perovskite crystal grains.
By systematically exploring the interfacial electronic structures
of individual perovskite grains, enhanced charge separation and reduced
back recombination were observed when an optimal design of interfacial
PbI<sub>2</sub> passivation layers consisting of a thickness less
than 20 nm at perovskite crystal grains was applied
Hysteresis-Free Contact Doping for High-Performance Two-Dimensional Electronics
Contact doping is considered crucial for reducing the
contact resistance
of two-dimensional (2D) transistors. However, a process for achieving
robust contact doping for 2D electronics is lacking. Here, we developed
a two-step doping method for effectively doping 2D materials through
a defect-repairing process. The method achieves strong and hysteresis-free
doping and is suitable for use with the most widely used transition-metal
dichalcogenides. Through our method, we achieved a record-high sheet
conductance (0.16 mS·sq–1 without gating) of
monolayer MoS2 and a high mobility and carrier concentration
(4.1 × 1013 cm–2). We employed our
robust method for the successful contact doping of a monolayer MoS2 Au-contact device, obtaining a contact resistance as low
as 1.2 kΩ·μm. Our method represents an effective
means of fabricating high-performance 2D transistors
Giant Photoresponse in Quantized SrRuO<sub>3</sub> Monolayer at Oxide Interfaces
The
photoelectric effect in semiconductors is the main mechanism
for most modern optoelectronic devices, in which the adequate bandgap
plays the key role for acquiring high photoresponse. Among numerous
material categories applied in this field, the complex oxides exhibit
great possibilities because they present a wide distribution of band
gaps for absorbing light with any wavelength. Their physical properties
and lattice structures are always strongly coupled and sensitive to
light illumination. Moreover, the confinement of dimensionality of
the complex oxides in the heterostructures can provide more diversities
in designing and modulating the band structures. On the basis of this
perspective, we have chosen itinerary ferromagnetic SrRuO<sub>3</sub> as the model material, and fabricated it in one-unit-cell thickness
in order to open a small band gap for effective utilization of visible
light. By inserting this SrRuO<sub>3</sub> monolayer at the interface
of the well-developed two-dimensional electron gas system (LaAlO<sub>3</sub>/SrTiO<sub>3</sub>), the resistance of the monolayer can be
further revealed. In addition, a giant enhancement (>300%) of photoresponse
under illumination of visible light with power density of 500 mW/cm<sup>2</sup> is also observed. Such can be ascribed to the further modulation
of band structure of the SrRuO<sub>3</sub> monolayer under the illumination,
confirmed by cross-section scanning tunneling microscopy (XSTM). Therefore,
this study demonstrates a simple route to design and explore the potential
low dimensional oxide materials for future optoelectronic devices