12 research outputs found
Substrate-Free Self-Assembly Approach toward Large-Area Nanomembranes
Free-standing two-dimensional nanostrucutures, such as graphene and semiconductor nanomembranes (NMs) featuring their integration with flexible polymer substrates, address applications in which electronic devices need to be stretchable or conformally positioned to nonplanar surfaces. We report a surfactant-directed surface assembly approach to producing large-area NMs at the water–air interface. The NMs were produced by employing the surfactants as templates as well as incorporating them in the crystal structures. By using excess amount of sodium dodecylsulfate (SDS), a tightly packed monolayer of dodecylsulfate (DS) ion was formed and directed the crystallization of submillimeter-sized zinc hydroxy dodecylsulfate (ZHDS) single-crystalline NMs over the entire water surface. This free-standing NM can be readily transferred to an arbitrary substrate and converted to ZnO <i>via</i> heat treatment. A flexible thin-film transistor was also fabricated using the transferred NMs and demonstrated reasonably good n-type transport properties. This approach circumvented the needs of single-crystalline substrates for making large-area NMs from materials that do not possess a laminate structure. It is a low-cost and large-scale synthesis technique and has great potential in developing NMs and flexible devices from various functional materials that are not feasible by conventional selective etching or delamination approaches
Cellulose Nanofibril/Reduced Graphene Oxide/Carbon Nanotube Hybrid Aerogels for Highly Flexible and All-Solid-State Supercapacitors
A novel type of highly flexible and
all-solid-state supercapacitor
that uses cellulose nanofibril (CNF)/reduced graphene oxide (RGO)/carbon
nanotube (CNT) hybrid aerogels as electrodes and H<sub>2</sub>SO<sub>4</sub>/polyÂ(vinyl alcohol) (PVA) gel as the electrolyte was developed
and is reported here. These flexible solid-state supercapacitors were
fabricated without any binders, current collectors, or electroactive
additives. Because of the porous structure of the CNF/RGO/CNT aerogel
electrodes and the excellent electrolyte absorption properties of
the CNFs present in the aerogel electrodes, the resulting flexible
supercapacitors exhibited a high specific capacitance (i.e., 252 F
g<sup>–1</sup> at a discharge current density of 0.5 A g<sup>–1</sup>) and a remarkable cycle stability (i.e., more than
99.5% of the capacitance was retained after 1000 charge–discharge
cycles at a current density of 1 A g<sup>–1</sup>). Furthermore,
the supercapacitors also showed extremely high areal capacitance,
areal power density, and energy density (i.e., 216 mF cm<sup>–2</sup>, 9.5 mW cm<sup>–2</sup>, and 28.4 μWh cm<sup>–2</sup>, respectively). In light of its excellent electrical performance,
low cost, ease of large-scale manufacturing, and environmental friendliness,
the CNF/RGO/CNT aerogel electrodes may have a promising application
in the development of flexible energy-storage devices
Graphene/Phase Change Material Nanocomposites: Light-Driven, Reversible Electrical Resistivity Regulation via Form-Stable Phase Transitions
Innovative
photoresponsive materials are needed to address the
complexity of optical control systems. Here, we report a new type
of photoresponsive nanomaterial composed of graphene and a form-stable
phase change material (PCM) that exhibited a 3 orders of magnitude
change in electrical resistivity upon light illumination while retaining
its overall original solid form at the macroscopic level. This dramatic
change in electrical resistivity also occurred reversibly through
the on/off control of light illumination. This was attributed to the
reversible phase transition (i.e., melting/recrystallization) behavior
of the microscopic crystalline domains present in the form-stable
PCM. The reversible phase transition observed in the graphene/PCM
nanocomposite was induced by a reversible temperature change through
the on/off control of light illumination because graphene can effectively
absorb light energy and convert it to thermal energy. In addition,
this graphene/PCM nanocomposite also possessed excellent mechanical
properties. Such photoresponsive materials have many potential applications,
including flexible electronics
Flexible Infrared Responsive Multi-Walled Carbon Nanotube/Form-Stable Phase Change Material Nanocomposites
Flexible
infrared (IR)-responsive materials, such as polymer nanocomposites,
that exhibit high levels of IR responses and short response times
are highly desirable for various IR sensing applications. However,
the IR-induced photoresponses of carbon nanotube (CNT)/polymer nanocomposites
are typically limited to 25%. Herein, we report on a family of unique
nanocomposite films consisting of multi-walled carbon nanotubes (MWCNTs)
uniformly distributed in a form-stable phase change material (PCM)
that exhibited rapid, dramatic, reversible, and cyclic IR-regulated
responses in air. The 3 wt % MWCNT/PCM nanocomposite films demonstrated
cyclic, IR-regulated on/off electrical conductivity ratios of 11.6
±
0.6 and 570.0 ± 70.5 times at IR powers of 7.3 and 23.6 mW/mm<sup>2</sup>, respectively. The excellent performances exhibited by the
MWCNT/PCM nanocomposite films were largely attributed to the IR-regulated
cyclic and reversible form-stable phase transitions occurring in the
PCM matrix due to MWCNT’s excellent photoabsorption and thermal
conversion capabilities, which subsequently affected the thickness
of the interfacial PCM between adjacent conductive MWCNTs and thus
the electron tunneling efficiency between the MWCNTs. Our findings
suggest that these unique MWCNT/PCM nanocomposites offer promising
new options for high-performance and flexible optoelectronic devices,
including thermal imaging, IR sensing, and optical communication
Cl-Doped ZnO Nanowires with Metallic Conductivity and Their Application for High-Performance Photoelectrochemical Electrodes
Doping
semiconductor nanowires (NWs) for altering their electrical and optical
properties is a critical strategy for tailoring the performance of
nanodevices. ZnO NWs grown by hydrothermal method are pervasively
used in optoelectronic, photovoltaic, and piezoelectric energy-harvesting
devices. We synthesized in situ Cl-doped ZnO NWs with metallic conductivity
that would fit seamlessly with these devices and improve their performance.
Possible Cl doping mechanisms were discussed. UV–visible absorption
spectroscopy confirmed the visible light transparency of Cl-doped
ZnO NWs. Cl-doped ZnO NW/TiO<sub>2</sub> core/shell-structured photoÂelectroÂchemical
(PEC) anode was fabricated to demonstrate the application potential
of highly conductive ZnO NWs. Higher photocurrent density and overall
PEC efficiency compared with the undoped ZnO NW-based device were
achieved. The successful doping and low resistivity of ZnO could unlock
the potential of ZnO NWs for applications in low-cost flexible transparent
electrodes
Optically Detected Magnetic Resonance for Selective Imaging of Diamond Nanoparticles
While there is great
interest in understanding the fate and transport
of nanomaterials in the environment and in biological systems, the
detection of nanomaterials in complex matrices by fluorescence methods
is complicated by photodegradation, blinking, and the presence of
natural organic material and other fluorescent background signals
that hamper detection of fluorescent nanomaterials of interest. Optically
detected magnetic resonance (ODMR) of nitrogen–vacancy (N<sub>V</sub>) centers in diamond nanoparticles provides a pathway toward
background-free fluorescence measurements, as the application of a
resonant microwave field can selectively modulate the intensity from
N<sub>V</sub> centers in nanodiamonds of various diameters in complex
materials systems using on-resonance and off-resonance microwave fields.
This work represents the first investigation showing how nanoparticle
diameter impacts the N<sub>V</sub> center lifetime and thereby directly
impacts the accessible contrast and signal-to-noise ratio when using
ODMR to achieve background-free imaging of N<sub>V</sub><sup>–</sup>nanodiamonds in the presence of interfering fluorophores. These results
provide new insights that will guide the choice of optimum nanoparticle
size and methodology for background-free imaging and sensing applications,
while also providing a model system to explore the fate and transport
of nanomaterials in the environment
Band-Bending of Ga-Polar GaN Interfaced with Al<sub>2</sub>O<sub>3</sub> through Ultraviolet/Ozone Treatment
Understanding
the band bending at the interface of GaN/dielectric under different
surface treatment conditions is critically important for device design,
device performance, and device reliability. The effects of ultraviolet/ozone
(UV/O<sub>3</sub>) treatment of the GaN surface on the energy band
bending of atomic-layer-deposition (ALD) Al<sub>2</sub>O<sub>3</sub> coated Ga-polar GaN were studied. The UV/O<sub>3</sub> treatment
and post-ALD anneal can be used to effectively vary the band bending,
the valence band offset, conduction band offset, and the interface
dipole at the Al<sub>2</sub>O<sub>3</sub>/GaN interfaces. The UV/O<sub>3</sub> treatment increases the surface energy of the Ga-polar GaN,
improves the uniformity of Al<sub>2</sub>O<sub>3</sub> deposition,
and changes the amount of trapped charges in the ALD layer. The positively
charged surface states formed by the UV/O<sub>3</sub> treatment-induced
surface factors externally screen the effect of polarization charges
in the GaN, in effect, determining the eventual energy band bending
at the Al<sub>2</sub>O<sub>3</sub>/GaN interfaces. An optimal UV/O<sub>3</sub> treatment condition also exists for realizing the “best”
interface conditions. The study of UV/O<sub>3</sub> treatment effect
on the band alignments at the dielectric/III-nitride interfaces will
be valuable for applications of transistors, light-emitting diodes,
and photovoltaics
Highly Stretchable Carbon Nanotube Transistors with Ion Gel Gate Dielectrics
Field-effect
transistors (FETs) that are stretchable up to 50%
without appreciable degradation in performance are demonstrated. The
FETs are based on buckled thin films of polyfluorene-wrapped semiconducting
single-walled carbon nanotubes (CNTs) as the channel, a flexible ion
gel as the dielectric, and buckled metal films as electrodes. The
buckling of the CNT film enables the high degree of stretchability
while the flexible nature of the ion gel allows it to maintain a high
quality interface with the CNTs during stretching. An excellent on/off
ratio of >10<sup>4</sup>, a field-effect mobility of 10 cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup>, and
a low
operating voltage of <2 V are achieved over repeated mechanical
cycling, with further strain accommodation possible. Deformable FETs
are expected to facilitate new technologies like stretchable displays,
conformal devices, and electronic skins
Enhanced Performance of Ge Photodiodes <i>via</i> Monolithic Antireflection Texturing and α‑Ge Self-Passivation by Inverse Metal-Assisted Chemical Etching
Surface
antireflection micro and nanostructures, normally formed
by conventional reactive ion etching, offer advantages in photovoltaic
and optoelectronic applications, including wider spectral wavelength
ranges and acceptance angles. One challenge in incorporating these
structures into devices is that optimal optical properties do not
always translate into electrical performance due to surface damage,
which significantly increases surface recombination. Here, we present
a simple approach for fabricating antireflection structures, with
self-passivated amorphous Ge (α-Ge) surfaces, on single crystalline
Ge (c-Ge) surface using the inverse metal-assisted chemical etching
technology (I-MacEtch). Vertical Schottky Ge photodiodes fabricated
with surface structures involving arrays of pyramids or periodic nano-indentations
show clear improvements not only in responsivity, due to enhanced
optical absorption, but also in dark current. The dark current reduction
is attributed to the Schottky barrier height increase and self-passivation
effect of the i-MacEtch induced α-Ge layer formed on top of
the c-Ge surface. The results demonstrated in this work show that
MacEtch can be a viable technology for advanced light trapping and
surface engineering in Ge and other semiconductor based optoelectronic
devices
Thin Film Receiver Materials for Deterministic Assembly by Transfer Printing
We present a specially designed materials
chemistry that provides
ultrathin adhesive layers with persistent tacky surfaces in solid,
nonflowable forms for use in transfer printing and related approaches
to materials and micro/nanostructure assembly. The material can be
photocured after assembly, to yield a robust and highly transparent
coating that is also thermally and electrically stable, for applications
in electronics, optoelectronics, and other areas of interest