Carbon, with its variety of allotropes and forms, is the most versatile material and virtually any combination of mechanical, optical, electrical or chemical properties can be achieved by controlling its structure and surface chemistry. While graphite, carbon fibers, glassy carbon, activated carbons, carbon black and diamond are widely used nowadays, fullerenes (also polymerized, endohedral and exohedral fullerides), carbon onions (multi-shell fullerenes), nanotubes (dozens of varieties), whiskers, nanofibers, cones, nanohorns, nanodiamonds and other nanoscale carbons have been attracting much attention in the past 20–30 years. Graphene is the latest example and is now the most widely researched.

There are already thousands of carbon nanomaterials to choose from, and we need different materials to meet a variety of performance requirements. It will be shown on an example of supercapacitor electrodes that 0D and 1D nanoparticles, such as onions and nanotubes, deliver very high power due to fast ion sorption/desorption on their outer surfaces. Two-dimensional graphene offers higher charge-discharge rates compared to porous carbons and high volumetric energy density. Three-dimensional porous activated, carbide-derived and templated carbon networks, having a high surface area and porosity in the Ångströms or nanometers range, can provide high energy density if the pore size is matched with the electrolyte ion size. Finally, carbon-based nanostructures further expand the range of nanomaterials available to us—recently discovered 2D transition metal carbides (MXenes) have already grown into a family with a dozen members in less than 3 years, and can challenge graphene in some applications.

In0.53Ga0.47As and atomic layer deposited (ALD) Al2O3 are among the candidates channel and dielectric materials, respectively, for future high performance III-V n-channel MOS devices. In particular, the ability to achieve large band offsets and a thermally stable interface with Al2O3 makes it an interesting choice for an interlayer dielectric between an InGaAs channel and higher-k materials. Achieving a low density of electrically active defects at the interface has been a long-standing challenge for all deposited dielectrics on III-V arsenide channels. Moreover, traps in the oxide layer may also reduce the charge in the channel and thus degrade the on-state performance of InGaAs MOSFET devices.

In this presentation, we describe approaches to passivate the interface and bulk oxide defects with various treatments, like large-dose exposure of the InGaAs surface to trimethyl-aluminum (TMA) prior to ALD, atomic hydrogen dosing, and either post-ALD or post-gate metal forming gas (5% H2/95% N2) anneals (FGA). Experimental methods employed include quantitative interface trap and oxide trap modeling[1, 2] of MOS capacitor data obtained over a range of frequencies and temperatures.

We also perform x-ray photoelectron spectroscopy to characterize possible film stoichiometry changes during annealing and the oxidation state of In, Ga and As at the dielectric/channel interface. These ex-situ data will be compared with the results of in-situ scanning tunneling microscopy/spectroscopy for certain passivation schemes[3, 4]. The effects of pre- and post-dielectric defect passivation schemes will be examined for ALD-Al2O3 samples prepared on both initially-clean and well-ordered As2-decapped In0.53Ga0.47As substrates and on initially air-exposed substrates. Relevant comparisons to low-temperature ALD-grown HfO2 films on InGaAs substrates will also be reported.

References:

1. H. Chen, Y. Yuan, B. Yu, J. Ahn, P.C. Mcintyre, P.M. Asbeck, M.J.W. Rodwell, and Y. Taur, IEEE Transactions on Electron Devices 59, 2383 (2012).

2. Y. Yuan, B. Yu, J. Ahn, P.C. Mcintyre, P.M. Asbeck, M.J.W. Rodwell, and Y. Taur, IEEE Transactions on Electron Devices 59, 2100 (2012).

3. W. Melitz, T. Kent, A.C. Kummel, R. Droopad, M. Holland, and I. Thayne, The Journal of Chemical Physics 136, 154706 (2012).

4. W. Melitz, J. Shen, T. Kent, R. Droopad, P. Hurley and A. C. Kummel, ESCS Transactions, 35(4) 175-189 (2011)

The emergence of electronic devices/systems with unprecedented functionalities mandatorily requires portable, flexible and sustainable power sources. Energy harvesting technology that can efficiently generate electricity from ambient environmental energy is the prerequisite for the realization of such new power sources. But due to their intrinsic limitation of unstable and uncontrollable electrical output, it is desirable to hybridize them with energy storage devices into single units, which could be capable of sustainably providing a stable output through utilizing the energy in the environment. For the electricity generation, mechanical energy is one of the best choices as energy source owing to its universally-existence in our living environment and human bodies.

A new type of devices—triboelectric nanogenerators (TENGs)—based on contact electrification has been recently invented as an effective and adaptive technology to generate electricity from motions. However, because the device structure and material properties have not been optimized, the output is still insufficient for sustainably driving electronic devices/systems.

Here, we demonstrated a rationally designed arch-shaped TENG as a flexible mechanical energy harvester with extremely-high power output. Through purposely introducing thermal stress on the surface, a flexible substrate was made into a naturally-bent shape, so that a steady gap formed between two triboelectric layers at strain-free conditions. This design facilitates the separation of the opposite tribo-charges, thus maximizes the electrical driving force.

This unique structure, together with the surface modification of tribo-layers, helps to largely enhance the output voltage, and power density to 230 V and 128 kW/m3, respectively, with an efficiency as high as 10~39%. For the first time, it realized the instantaneous driving of tens of regular electronic devices (LEDs), and also the fully charging of a Li-ion battery.

[1] For the development of sustainable power sources, we further hybridized the arch-shaped TENG with a flexible Li-ion-battery into as a single device—a flexible self-charging power unit (SCPU), which allows a battery to be charged directly by ambient mechanical motion. This physical hybridization enables a new operation mode: the “sustainable mode”, in which the environmental mechanical energy is scavenged to charge the battery while the battery keeps driving an external load.

In this mode, the demonstrated SCPU can provide a continuous and sustainable DC current of 2 µA at a stable voltage of 1.55 V for as long as there is mechanical motion/agitation. It can be used to continuously drive a UV sensor for extended period of time. Thus, the SCPU can serve as an independent and sustainable power unit, which will meet the general requirement of almost any electronic device. [2]

[1] Wang, S. H.; Wang, Z. L. et al. Nano Lett. 2013, 13, 2226-2233.

[2] Wang, S. H.; Wang, Z. L. et al. ACS Nano Under review.

Graphene is a one-atom-thick material with unusual and highly promising for applications electrical [1] and thermal [2] properties. First obtained by mechanical exfoliation from graphite, graphene is now efficiently grown by chemical vapor deposition (CVD) on copper (Cu) films. It was reported that layered graphene - metal composites have enhanced mechanical strength. However, it was not known how deposition of graphene on Cu films affects their thermal properties.

In this talk we will report of our investigation of thermal properties of graphene coated Cu films. The measurements were performed using the modified “laser flash” technique, which allowed for investigation of the in-plane heat conduction properties. It was found hat CVD of graphene enhances the thermal diffusivity and thermal conductivity of graphene coated Cu films. Deposition of graphene increased the thermal conductivity K of 9-μm (25-μm) thick Cu films by up to 24% (16%) near the room temperature.

Interestingly, the increase of thermal conductivity of graphene coated Cu films is primarily due to changes in Cu morphology during graphene deposition and associated with it temperature treatment. Graphene’s action as an additional heat conducting channel was small due to its small thickness as compared to that of Cu films. Enhancement of thermal properties of metal films via graphene coating may lead to major changes in metallurgy and graphene applications in hybrid graphene - Cu interconnects in Si complementary metal-oxide-semiconductor (CMOS) technology.

[1] K.S. Novoselov et al. “Electric field effect in atomically thin carbon films,” Science, 306, 666 (2004);

[2] A.A. Balandin, et al., “Superior thermal conductivity of single-layer graphene," Nano Letters, 8, 902 (2008).

The work at UC Riverside was supported, in part, by the National Science Foundation (NSF) project ECCS-1307671 on engineering thermal properties of graphene, by DARPA Defense Microelectronics Activity (DMEA) under agreement number H94003-10-2-1003, and by STARnet Center for Function Accelerated nanoMaterial Engineering (FAME) - Semiconductor Research Corporation (SRC) program sponsored by MARCO and DARPA.

Replacing silicon with high-mobility channel materials such as InGaAs will be surely the next evolution of MOSFET devices. Alternative materials such as High-k dielectrics and metal gates have already been successfully introduced. InGaAs based channels hold the promise of circuits operating at lower Vdd and hence consuming low power as the dynamic power roughly scales as V2dd. Two different strategies for integrating As based compounds as MOSFET channels are actually foreseen: fully depleted III-V/On Insulator or FinFET. This integration still faces many challenges like direct III-V epitaxy on Si, channel/high k interface control, and contact resistance.

We focus on the direct growth of As compounds on Si(100) 300 mm substrates. GaAs and InGaAs layers are grown by an AMAT MOCVD tool. TMIn, TMGa and TMAl are used as group III elemental precursors whereas TBAs is used as group V elemental precursor. Typical growth temperature ranges between 300 and 700°C and pressure ranges between 1 and several hundred Torr. We have studied the structural and the physical properties of GaAs, InGaAs, AlGaAs layers grown either on blanket or patterned Si(100) wafers by AFM, FIBSTEM, TEM, SIMS, µPL, cathodoluminescence, XRD. We showed an improvement of the material quality as they are elaborated in SiO2 cavity even with an aspect ratio less than 2. Antiphase boundary and dislocation densities are strongly decreased as the width of the cavity goes bellow 100 nm. By adjusting properly the growth conditions and the stack in the quantum well structure, we were able to observe room temperature micro-photoluminescence of single InGaAs QW, with In composition ranging between 10 and 53% and a total stack thickness well below 1 µm.

Even though extraordinary progress has been made in the field of Si and Cu(In,Ga)(S,Se)2 (CIGS) solar cells/modules over the years and their performance significantly improved, fabrication/material costs and safety still remain major concerns. In terms of material design for compound semiconductors, the most cost-effective, safe, and easy-handling anions are sulfur and oxygen. Nowadays, to obtain compound semiconductors by reacting inexpensive metal cations (precursors) with gas-phase anions has become a significant issue. For realizing this, sulfurization and oxidation are appropriate physical processes because they are based on simple thermal diffusion of anions into a metal precursor. In fact, sulfurization/selenization techniques are adopted for commercially manufacturing CIGS solar modules because of the cost effectiveness, scalability, and uniformity of such techniques.

Tin monosulfide (SnS) has a direct energy bandgap of 1.3 eV and a high optical absorption coefficient of 104 cm−1. Therefore, SnS is perceived to be a promising candidate as a cost-effective and earth-abundant inorganic material for use in the fabrication of next-generation solar cells. Although the theoretical conversion efficiency of SnS-based solar cells is high, the demonstrated efficiencies of such cells are still low. Some of the reasons for low efficiencies are considered to be the poor crystal quality of a SnS layer, and the mismatch in the band diagram at a pn-heterojunction. In fact, most researchers simply refer to the CIGS solar cells structure. The growth mechanism of SnS or the appropriate band offset of SnS-based solar cells has not yet been clarified because it shows metastable phase.

Wide-bandgap transparent conducting oxide (TCO) films exhibit only n-type conductivity. On the other hand, NiO shows only p-type conductivity, and is composed of inexpensive and less toxic elements. Recently, NiO has also been used in visibly transparent solar cells, which are attractive because their optical transparency permits greater flexibility in terms of installation locations. In addition, NiO films can be easily obtained just by the oxidation of Ni. We have proposed a “NiO-based invisible sensor” that receives electrical power from a “NiO-based invisible solar cell”. This sensor can be fixed on a wall or ceiling without a wired connection. Moreover, the user will not be aware of these invisible sensors and will not experience the stress associated with being monitored. Therefore, there is a great market opportunity for various environments, and the application possibilities for invisible devices based on NiO-based solar cells are limited only by imagination.

In this presentation, we will introduce some earth-abundant solar absorbers (SnS, Cu2SnS3, Cu2O, and related compounds) and TCOs (NiO, CuAlO2, and related compounds) for use in photovoltaics, with focus on materials research and device development, in terms of the most recent advances and experimental results.

Copper nanowire (CuNW) networks are one of the most promising candidates to replace indium tin oxide films as the premier transparent conducting electrode (TCEs) due to its high sheet resistance(Rs) -transmittance(T) performance, superior mechanical flexibility and low lost. However, the chemical activity of CuNWs causes a substantial increase in the Rs after thermal oxidation or chemical corrosion, which may undermine its applicability. In this work, we utilize atomic layer deposition(ALD) to coat a passivation layer onto electrospun copper nanowires and remarkably enhance their durability. The passivation layer is composed of 20-nm-thick aluminum-doped zinc oxide (AZO) for the inner layer and 1-nm-thick aluminum oxide for the outer layer. Without changing the optical transmittance, the passivated CuNW TCE shows an resistance increase of only 10% after thermal oxidation at 160°C in dry air and 80°C in humid air with 80% relative humidity, whereas the bare CuNWs quickly become insulating. In addition, the coating and baking of the acidic PEDOT:PSS layer increases the Rs of bare CuNW by 6 orders of magnitude, while the passivated CuNWs show an 18% increase. Our work demonstrates that this ALD method can greatly enhance the reliability of CuNW TCE and thus provide a practical solution for the degradation problem of metal nanowire TCEs.

Graphene, the atomic thin carbon film with honeycomb lattice, holds great promise in a wide range of applications, arising from its unique band structure and excellent electronic, optical, mechanical and thermal properties. The research of this star material is being stimulated by the development of various emerging preparation techniques, among which chemical vapor deposition (CVD) has received the fastest advances in the last few years. This talk focuses on our recent progresses towards the controlled surface growth of graphene and its two-dimensional (2D) hybrids via CVD process engineering. The general strategy is to design and control the elementary steps of catalytic CVD process for achieving a precise control of layer thickness, stacking order, domain size, doping and energy band structure. A particular emphasis is laid on the design of growth catalysts, including bimetal alloys and groups IVB-VIB transition metal carbides. The sp2 carbon network of graphene is chemically very stable and hence it is a great challenge for its chemical doping and tailoring. We are working with a photochemical approach for graphene chemistry, where the chemical scissors are the highly reactive radicals generated from photochemical processes. A number of examples are given, including photochlorination, photomethylation, photocatalytic oxidation and Janus chemistry.

References:

1) LM Zhang, JW Yu, MM Yang, Q Xie, HL Peng, ZF Liu, Janus graphene from asymmetric two-dimensional chemistry, Nature Comm., 4(2013) 1443-1449.

2) W Yan, WY He, ZD Chu, MX Liu, L Meng, RF Dou, YF Zhang, ZF Liu, JC Nie, L He, Strain and curvature induced evolution of electronic band structures in twisted graphene bilayer, Nature Comm., 4(2013) 2159-2165.

3) K Yan, D Wu, HL Peng, L Jin, Q Fu, XH Bao, ZF Liu, Modulation-doped growth of mosaic graphene with single-crystalline p-n junctions for efficient photocurrent generation, Nature Comm., 3(2012) 1280-1286.

4) BY Dai, L Fu, ZY Zou, M Wang, HT Xu, S Wang, ZF Liu,Rational design of a binary metal alloy for chemical vapour deposition growth of uniform single-layer graphene, Nature Comm., 2(2011) 522-527.

5) K Yan, L Fu, HL Peng, ZF Liu, Designed CVD Growth of Graphene via Process Engineering, Acc. Chem. Res., 10(2013) 2263-2274.

As “smaller is stronger”, nanostructured materials such as nanowires, nanotubes, nanoparticles, thin films, atomic sheets etc. can withstand non-hydrostatic (e.g. tensile or shear) stresses up to a significant fraction of its ideal strength without inelastic relaxation by plasticity or fracture.

Large elastic strains can be generated by epitaxy, or by static or dynamical external loading on small-volume materials, and can be spatially homogeneous or inhomogeneous. This leads to new possibilities for tuning the physical and chemical (e.g. electronic, optical, magnetic, phononic, catalytic, etc.) properties of a material, by varying the 6-dimensional elastic strain as continuous variables. By controlling the elastic strain field statically or dynamically, one opens up a much larger parameter space for optimizing the functional properties of materials, which gives a new meaning to Feynman’s 1959 statement “there's plenty of room at the bottom”.

The roadmap for rational ESE will be addressed. These include precisely applying and measuring large elastic strain (AFM, nanomechanics, microscopy and spectroscopy), predicting what strain does to physical and chemical properties (ab initio to continuum scale modeling), tailoring (sometimes via in situ experiments) quantitatively the properties in desired directions, and understanding how large an elastic strain can be sustained for how long (mechanisms of plastic deformation, defect evolution and failure in small-volume materials).

[1] Zhu, Li, "Ultra-strength materials," Progress in Materials Science 55 (2010) 710-757.

[2] J. Feng, Qian, Huang, Li, "Strain-engineered artificial atom as a broad-spectrum solar energy funnel," Nature Photonics 6 (2012) 865-871.

[3] Hao et al, "A Transforming Metal Nanocomposite with Large Elastic Strain, Low Modulus, and High Strength," Science 339 (2013) 1191-1194.