themes and objectives
research efforts bridge various science and engineering disciplines,
and are primarily focused on engineering functional nanomaterials for
technological applications. Below is a list of some of the active
projects in our group:
performance and flexible photovoltaics
represents one of the most abundant and yet least harvested source of
renewable energy. In recent years, tremendous progress has been
made in developing photovoltaics (PVs) that can be potentially mass
employed. Of particular interest to cost-effective solar cells is to
utilize novel device structures and materials processing for enabling
acceptable efficiencies. In this regard, we have recently demonstrated
the direct growth of highly regular, single crystalline nanopillar
(NPL) arrays of optically active semiconductors on aluminum substrates
which are then configured as solar cell modules. As an example, we have
demonstrated a PV structure that incorporates 3D, single crystalline
n-CdS NPLs, embedded in poly-crystalline thin films of p-CdTe, to
enable high absorption of light and efficient collection of the
carriers. Through experiments and modeling, we have demonstrated the
potency of this approach for enabling highly versatile solar modules on
both rigid and flexible substrates with enhanced carrier collection
efficiency arising from the geometric configuration of the NPLs.
So far, materials conversion efficiency of ~12% with a device
conversion efficiency of ~6% (50% optical transparency loss from the
unoptimized top contact) are attained in our first generation solar
modules. This approach, which is compatible with a wide range of
materials, could potentially have a large impact in the realization of
efficient and light weight solar panels.
Video (~3 MB): The open circuit voltage is measured
light illumination as the flexible solar module is bent, showing
1. Z. Fan,
Razavi, J. Do, A. Moriwaki, O. Ergen, Y.-L. Chueh, P. W. Leu, J. C. Ho,
T. Takahashi, L. A. Reichertz, S. Neale, K. Yu, M. Wu, J. W. Ager, A.
Javey. "Three dimensional nanopillar array photovoltaics on low cost
and flexible substrates", Nature
Materials, 8, 648-653, 2009.
Ming Wu, Dr.
II. Large-area printed inorganic
electronics and sensors
In recent years,
there has been tremendous progress in the research and development of
printable electronics on mechanically flexible substrates based on
inorganic active components, which provide high performances and stable
device operations at low costs. In this regard, various approaches have
been developed for the direct transfer or printing of micro- and
nanoscale, inorganic semiconductors on substrates. In one specific
direction, we have recently developed the process technology for large
scale contact and roll printing of semiconductor nanowire parallel
arrays on substrates with high uniformity. The process is compatible
with a wide range of nanowire materials/diameters, and receiver
substrates, including plastics, paper, and glass. The printed
nanowire arrays can then be readily configured as the active channel
material for a wide range of high performance electronic and sensor
device structures. Devices with
carrier mobilities as high as 700 cm2/Vs are enabled by
using this process, enabling the exploration of a wide range of circuit
structures unreachable before with the printed organic
electronics. Furthermore, the simplicity and low thermal
requirements of the process allows for heterogeneous integration of
nanowire materials with different functionalities on substrates through
a multi-step printing process. As a proof of concept, a simple
imager has been developed based on this approach.
1. Z. Fan,
J. C. Ho, Z. A. Jacobson, H. Razavi, A. Javey, "Large Scale,
Heterogeneous Integration of Nanowire Arrays for Image Sensor
Circuitry", Proceedings of the National Academy of Sciences (PNAS),
105, 11066–11070, 2008.
2. Z. Fan,
J. C. Ho, T. Takahashi, R. Yerushalmi, K. Takei, A. C. Ford,
Y.-L. Chueh, A. Javey. "Towards the Development of Printable Nanowire
Electronics and Sensors," Advanced Materials, in press, 2009.
Takahashi, K. Takei, J. C. Ho, Y.-L. Chueh, Z. Fan, A. Javey.
"Monolayer Resist for Patterned Contact Printing of Aligned Nanowire
Arrays," Journal of the American Chemical Society, 131 (6), 2102-2103,
4. Z. Fan,
J. C. Ho, Z. A. Jacobson, R. Yerushalmi, R. L. Alley, H.
Razavi, A. Javey, "Wafer-Scale Assembly of Highly Ordered Semiconductor
Nanowire Arrays by Contact Printing", Nano Letters, 8(1), 20-25, 2008.
Yerushalmi, Z. A. Jacobson, J. C. Ho, Z. Fan, A. Javey, “Large
scale, highly ordered assembly of nanowire parallel arrays by
differential roll printing”, Applied Physics Letters, 91, 203104, 2007.
Nanoscale processing and fabrication through the use of molecular
III.(a) sub 5nm doping of semiconductors via molecular monolayers
One of the major challenges towards
scaling of the electronic devices to the nm-regime is attaining
controlled doping of semiconductor materials with atomic accuracy as at
such small scales, the various existing technologies suffer from a
number of setbacks. In this regard, we have developed a novel strategy
for controlled, nanoscale doping of semiconductor materials by taking
advantage of the crystalline nature of silicon and its rich,
self-limiting surface reaction properties. Our method relies on the
formation of highly uniform and covalently bonded monolayer of dopant
containing molecules which allows for deterministic positioning of
dopant atoms on the Si surfaces. In a subsequent annealing step, the
dopant atoms are diffused into the Si lattice to attain the desired
doping profile. We have shown the versatility of our monolayer
doping (MLD) approach through controlled p- and n-doping of a wide
range of semiconductor materials, including ultrathin SOI and
nanowires, which are then configured into novel transistor structures.
The dopant dose is controlled by engineering the molecular structure of
the dopant precursor with the larger molecules resulting in lower dose.
Notably, sub 5nm junctions with high areal dopant dose and 70-80%
electrically active dopant contents are enabled, even for fast
diffusing dopants, such as phosphorous by the use of MLD and the
conventional spike annealing.
C. Ho, R. Yerushalmi, Z. A. Jacobson, Z. Fan, R. L. Alley, A. Javey,
"Controlled nanoscale doping of semiconductors via molecular
monolayers", Nature Materials,
7 (1), 62-67, 2008.
C. Ho, R. Yerushalmi, G. Smith, P. Majhi, J. Bennett, J. Halim, V.
Faifer, A. Javey. "Wafer-Scale, Sub-5 nm Junction Formation by
Monolayer Doping and Conventional Spike Annealing", Nano Letters, 9
(2), 725–730, 2009.
3. J. C. Ho, A. C.
Ford, Y.-L. Chueh, P. Leu, O. Ergen, K. Takei, G.
Smith, P. Majhi, J. Bennett, A. Javey. "Nanoscale doping of InAs via
sulfur monolayers", Applied Physics
Letters, 95, 072108, 2009.
Nanoscale electron transport physics and devices of high mobility
the materials are miniaturized to nanoscale, their electrical and
optical properties are tailored, arising from a number of size and
shape dependent factors. We are interested to explore and
understand the basic transport properties and device physics of
materials as function of their dimensions at nanoscale.
particular interest is the dependence of the carrier mobility on
nanowire (NW) radius for a given material, especially since smaller NWs
are highly attractive for the channel material of nanoscale transistors
as they enable improved electrostatics and lower leakage currents. Most
theoretical studies have found carrier mobility to increase with radius
for sub-10 nm Si NWs, either attributing the trend to the dominant
surface roughness scattering in smaller radius NWs, or an enhanced
phonon scattering rate due to an increased electron-phonon wavefunction
overlap in smaller radius NWs. On the other hand, experimental reports
in the literature have been contradictory, ranging from observation of
mobility enhancement to degradation with Si nanowire
miniaturization for diameters (or widths) down to 10 nm. Therefore, the
diameter dependency of the mobility highly depends on the specific
nanowire material system, the diameter range, and the method used to
assess the electron mobility. The challenge in attaining accurate
experimental data mainly arises from the difficulty of ohmic contact
formation to nanoscale materials and the direct measurement of the gate
capacitance. In this regard, we have recently reported the
detailed current-voltage (I-V) and capacitance-voltage (C-V)
spectroscopy of individual InAs NWs with ohmic contacts at different
temperatures; therefore, enabling the direct assessment of field-effect
mobility as a function of NW diameter while elucidating the role of
surface/interface fixed charges and trap states on the electrical
properties. The field-effect mobility is found to monotonically
decrease as the radius is reduced to sub-10 nm, with the low
temperature transport data clearly highlighting the drastic impact of
the surface roughness scattering on the mobility degradation for
miniaturized nanowires. More generally, our presented approach
may serve as a versatile platform for in-depth characterization of
nanoscale, electronic materials.
A. Ford, J. C. Ho, Y.-L. Chueh, Y.-C.
Tseng, Z. Fan, J. Guo, J. Bokor, A. Javey. "Diameter-Dependent Electron
Mobility of InAs Nanowires", Nano
Letters, 9 (1), 360-365, 2009.
Y.-L. Chueh, A. C. Ford, J. C. Ho, Z.
A. Jacobson, Z. Fan, C.-Y. Chen, L.-J. Chou, and A. Javey. “Formation
and Characterization of NixInAs/InAs Nanowire Heterostructures by Solid
Source Reaction.” Nano Letters,
8 (12), 4528–4533, 2008.
Jeff Bokor and
Professor Jing Guo
V. The power of nano: programmable &
multifunctional connectors based on NW arrays
The evolution of biological systems
has generated hierarchical nano- and microfibrillar structures with
diverse mechanical, optical, and sensing functionalities. These
excellent and unique structure-related properties found in biological
systems have inspired researchers to generate artificial materials
mimicking, for example, the amazing adhesion abilities of gecko’s foot,
and superhydrophobic surface enabling self-cleaning of dirt by rolling
a water droplet in lotus leaf. Especially, the nanofibrillar structures
of synthetic gecko adhesives enable binding to almost any surface by
van der Waals (vdW) interactions. Recently, we reported self-selective
connectors based on nanofibrillar structures that enable efficient
binding with self-similar surfaces but weak adhesion towards other
surfaces in both dry and wet environments. The connectors consist of
inorganic/organic, core/shell nanowire (NW) forests with the inorganic
core serving as the rigid backbone and the organic shell providing a
soft surface for conformal contact. Since the hybrid NWs are relatively
stiff as compared to those used in most gecko adhesives, minimal
adhesion is observed when brought in contact with flat surfaces (i.e,
they are poor adhesives). However, the contact area, and
therefore, the vdW interactions are drastically enhanced (~1000x
enhancement) when the NW forests are engaged with self-similar
surfaces, resulting in high shear strength with relatively low
engagement/disengagement forces. This selective binding with
self-similar surfaces presets a major difference between a connector
and an adhesive. As compared to the Velcro technology, NW connectors
operate via chemical interactions, making them high scalable at both
macro and micro-scales. Furthermore, they have soft surfaces, are
ultrathin (only 10 um in thickness) and show minimal adhesion even on
porous foreign surfaces, such as cloth.
macroscopic shear adhesion strength (~163 N/cm2) with
to non self-similar surfaces, anisotropic adhesion behavior (shear to
normal strength ratio ~25), reusability (~98 attach/detach cycles), and
self-cleaning of the surface from contaminant particles, similar to the
lotus effect. One of
the advantages of hybrid core/shell NW connectors is their
tunable multi-functionalities depending on the composition of the
hybrid structures. For instance, we have demonstrated connectors with
electrical connectivity and physical binding by using metallic outer
shells on the hybrid NW connectors. The NW electrical connectors may
enable the exploration of a wide range of applications involving
reversible assembly of micro- and macro-scale components with built-in
Video 1 (~3 MB):
NW connectors attached to the ends of a rubber belt are shown in action.
Supplementary Video 2
NW electrical connectors, exhibiting both physical adhesion and
electrical connectivity, are utilized to attach an array of LEDs to a
battery pack mounted on the wall.
Video 3 (~3 MB):
Lotus effect - hybrid NW forests exhibit superhydrophobic surface
behavior, enabling the self-cleaning of the surface contaminants (in
this case, sand particles) by rolling of a water droplet.
J. Lee, B. E. Schubert, Y.-L. Chueh, P. W. Leu, R. S. Fearing, A.
Javey. "Hybrid Core-Shell Nanowire Forests as Self-Selective Chemical
Connectors," Nano Letters, 9
(5), 2054–2058, 2009.
2. R. Kapadia, H. Ko,
Y.-L. Chueh, J. Ho, T. Takahashi, Z. Zhang, A. Javey. "Hybrid
core-multi-shell nanowire forests for electrical connector
applications," Applied Physics Letters,
94, 263110, 2009.
3.H. Ko, Z. Zhang, Y.-L. Chueh, J. C. Ho, J. Lee, R. S. Fearing, and A.
Javey. "Wet and dry adhesion properties of self-selective nanowire
connectors", Advanced Functional
Materials, in press, 2009.
Collaborators: Professor Ron Fearing