Carlo CARRARO | UC Berkeley (original) (raw)

Papers by Carlo CARRARO

Although much insight has been gained into the problem of stiction for MEMS operating in air, no ... more Although much insight has been gained into the problem of stiction for MEMS operating in air, no systematic investigation of the phenomenon in microfluidic environments has been performed. With the increased interest in BioMEMS and microfluidic applications, the characterization of microstructure adhesion in fluids becomes important. The adhesion characteristics of oxideterminated and SAM-coated polycrystalline Si (polysilicon) cantilever beams in various solvents are investigated. It is found that surfaces behave quite differently in microfluidic environments, when compared to their behavior in air. Oxide-terminated surfaces are found to exhibit much reduced adhesion in water, in comparison to air. In contrast, hydrophobic SAM-coated surfaces experience strong adhesion in water, a behavior opposite to what is observed in air. It is also observed that oxide-terminated surfaces exhibit greater adhesion in hydrocarbons, compared to SAMcoated surfaces, while surfaces show no adhesion in isopropyl alcohol regardless of surface termination.

Cement and Concrete Research, 2021

Creep of the cement matrix affects the structural stability of concrete. In Portland cements, the... more Creep of the cement matrix affects the structural stability of concrete. In Portland cements, the creep is largely controlled by the binding phase calcium-(aluminum-)silicate-hydrate, or C-(A-)S-H. This phase has a lamellar structure and under deviatoric stress aligns its c-axis with the principal stress. However, the limiting resistance to this reorientation is unknown at the nanocrystalline level. Small-angle X-ray scattering shows that the lamellae thickness decreases under 100's MPa deviatoric stress. Deviatoric stress Raman spectroscopy shows that there are two ways that this break-up can occur. If the material's silicate chains are cross-linked, then strain in Si-O bonds does not increase above certain stresses, indicating a relaxation adjacent to the Si-O bond. If the chains are not cross-linked, then the silicate chains are broken up by rastering against each other, introducing defects. These results show that the plastic deformation of C-(A-)S-H is relevant for Portland cement creep. Table of contents image Material strain Si-O-Ca bond strain Calciumsilicate hydrate Raman laser Turbostratic slip observed! Deviatoric-stress Raman spectroscopy shows how the structure of cement hydrates affects the way they plastically deform.

Chemistry – A European Journal, 2019

Ac lassic challenge in chemical sensingi ss electivity.M etal-organic frameworks (MOFs) are an ex... more Ac lassic challenge in chemical sensingi ss electivity.M etal-organic frameworks (MOFs) are an exciting class of materials because they can be tuned for selective chemical adsorption. Adsorptione ventst riggerw ork-function shifts, which can be detectedw ith ac hemical-sensitive field-effect transistor (power % microwatts). In this work, several case studiesw ere used towards generalizing the sensing mechanism, ultimately towards our metal-centric hypothesis. HKUST-1 was used as ap roof-of-principle humidity sensor. The response is thickness independent, meaning the response is surfacel ocalized. ZIF-8 is demonstrated to be an NO 2-sensing material,a nd the response is dominated by adsorptiona tm etal sites. Finally, MFM-300(In) shows how standard hard-soft acid-base theory can be used to qualitatively predict sensorr esponses. This paper sets the groundwork for using the tunability of metal-organic frameworks for chemicalsensing with distributed, scalable devices.

Cement and Concrete Research, 2019

Calcium (alumino)silicate hydrate (C-(A-)S-H) is the main binding phase in blended cement concret... more Calcium (alumino)silicate hydrate (C-(A-)S-H) is the main binding phase in blended cement concrete. Understanding the chemistry and structure of C-(A-)S-H is essential to optimizing concrete properties such as compressive strength and durability; yet questions remain around the coordination environments of Ca and Al in its structure with various chemical compositions and equilibration temperatures.

ACS Nano, 2016

Transition metal dichalcogenides (TMDCs) have been extensively explored for applications in elect... more Transition metal dichalcogenides (TMDCs) have been extensively explored for applications in electronic and optoelectronic devices due to their unique material properties. However, the presence of large contact resistances is still a fundamental challenge in the field. In this work, we study defect engineering by using a mild plasma treatment (He or H 2) as an approach to reduce the contact resistance to WSe 2. Material characterization by Xray photoelectron spectroscopy, photoluminescence, and Kelvin probe force microscopy confirm defect-induced n-doping, up to degenerate level, which is attributed to the creation of anion (Se) vacancies. The plasma treatment is adopted in the fabrication process flow of WSe 2 n-type metal-oxide−semiconductor field-effect transistors to selectively create anion vacancies at the metal contact regions. Due to lowering the metal contact resistance, improvements in the device performance metrics such as a 20× improvement in ON current and a nearly ideal subthreshold swing value of 66 mV/dec are observed. This work demonstrates that defect engineering at the contact regions can be utilized as a reliable scheme to realize high-performance electronic and optoelectronic TMDC devices.

ECS Transactions, 2014

Graphene grown by CVD on Cu foils has generated interest due to low cost and the prospect of larg... more Graphene grown by CVD on Cu foils has generated interest due to low cost and the prospect of large-area monolayer coverage. The initial nucleation and growth dynamics of graphene play a critical role in determining the final film quality. In this work, we investigate the effects of the electrochemical synthesis onto graphene quality, showing the influence of the Cu-Ru interdiffusion. The growth of good quality graphene layers is also discussed in terms of the role played by grain boundaries and diffusion at the grain boundaries. This comparison shows that the graphene having the best quality is produced using a ruthenium sandwich covering a copper core. This setting avoids contaminations from evaporation and recondensation of copper during thermal cycling due to the CVD process.

Advanced Materials Interfaces, 2017

A high‐performance NO2 sensor based on the 3D MoS2 aerogel is presented. Compared to single‐ or f... more A high‐performance NO2 sensor based on the 3D MoS2 aerogel is presented. Compared to single‐ or few‐layer MoS2, 3D assemblies of 2D MoS2 provide more surface area per footprint with a simple and scalable synthesis. Integration of the 3D MoS2 aerogel on a low‐power microheater platform is demonstrated, and the sensing behavior of the 3D MoS2 aerogel is investigated. A two‐step sulfurization treatment is developed to obtain a high‐quality MoS2 aerogel with strong sensing performance. The aerogel exhibits low detection limit (50 ppb) toward NO2 at room temperature, while after the two‐step sulfurization treatment, it also exhibits fast response and recovery at low heater temperature of 200 °C, with no decrease in sensitivity. The observed p‐type sensing behavior of MoS2 aerogel is investigated and identified as being controlled by the defect state (as probed by the S:Mo ratio). It is demonstrated that annealing in a hydrogen environment changes the defect state of the MoS2 aerogel by c...

Journal of Crystal Growth, 2006

In this paper, nitrogen-doped 4H-Silicon carbide (SiC) epilayers are grown on 4° off-axis 〈0001¯〉... more In this paper, nitrogen-doped 4H-Silicon carbide (SiC) epilayers are grown on 4° off-axis 〈0001¯〉 C-face 4H-SiC substrates by horizontal hot-wall chemical vapor deposition. Surface morphology of the epilayers shows a strong dependence on N2 flow, C/Si ratio and growth temperature. Surface defect densities are higher for samples grown under high C/Si ratio and high N2 flow condition. Lightly doped epilayers

Physical Review B, 2009

The temperature dependence of the Stokes first-order optical phonon frequency has been measured f... more The temperature dependence of the Stokes first-order optical phonon frequency has been measured for individual silicon nanowires with diameters between 33 to 180 nm in the temperature range of 20 to 300°C. The nanowires were synthesized via both the vapor-liquid-solid method and electrochemical etching of bulk silicon. Significant laser-induced local heating was avoided by using a laser power of 0.5 mW or less, corresponding to fluxes of Յ0.7 mW/ m 2. For both types of nanowires the slope of Raman frequency vs temperature closely matches the value of bulk Si, ͑ d dT ͒ = −0.022Ϯ .001 cm −1°C−1 , across the entire diameter range, indicating no change in lattice anharmonicity. These results have important implications for understanding nanowire lattice thermal conductivity and extending the domain for Raman thermometry of silicon nanostructures.

The Journal of Physical Chemistry Letters, 2018

The behavior of composite nanostructures depends on both size and elemental composition. Accordin... more The behavior of composite nanostructures depends on both size and elemental composition. Accordingly, concurrent control of size, shape, and composition of nanoparticles is key to tuning their functionality. In typical core−shell nanoparticles, the high degree of symmetry during shell formation results in fully encapsulated cores with severed access to the surroundings. We commingle light parameters (wavelength, intensity, and pulse duration) with the physical properties of nanoparticles (size, shape, and composition) to form hitherto unrealized core-vest composite nanostructures (CVNs). Unlike typical core−shells, the plasmonic core of the resulting CVNs selectively maintains physical access to its surrounding. Tunable variations in local temperature profiles ≳50°C are plasmonically induced over starburstshaped nanoparticles as small as 50−100 nm. These temperature variations result in CVNs where the shell coverage mirrors the temperature variations. The precision thus offered individually tailors access pathways of the core and the shell.

Physical Review E, 1997

We study the statistical mechanics and dynamics of crystalline films with a fixed internal connec... more We study the statistical mechanics and dynamics of crystalline films with a fixed internal connectivity on a random substrate. Defect free triangular lattices exhibit a sharp transition to a low temperature glassy phase with anomalous phonon fluctuations and a nonlinear force-displacement law with a continuously variable exponent, similar to the vortex glass phase of directed lines in 1+1 dimensions. The periodicity of the tethered monolayer acts like a filter which amplifies particular Fourier components of the disorder. However, the absence of annealed topological defects like dislocations is crucial: the transition is destroyed when the constraint of fixed connectivity is relaxed and dislocations are allowed to proliferate. 46.30Pa 68.35Rh 76.60Ge Typeset using REVT E X

Microelectromechanical Systems, 2003

The unique operating conditions of micro-thermochemical systems introduce many materials compatib... more The unique operating conditions of micro-thermochemical systems introduce many materials compatibility issues that must be addressed, particularly where thin film coatings are concerned. These issues include oxidation, wear, friction, and thermal stability. This work aims to explore the effectiveness of polycrystalline 3C-SiC films deposited by low temperature chemical vapor deposition on silicon substrates as a means for the remediation of these effects. The chemical structure of the deposited films is examined by X-ray photoelectron spectroscopy and X-ray diffraction techniques. Surface physical characteristics are evaluated by atomic force microcopy as well as by scanning electron microscopy. The structural makeup of these films is characterized both optically and by the sample’s resistance to chemical etching. These barrier films are found to conformally deposit and can therefore be used to coat substrates or individual components, as well as to create highly environmentally res...

Tribology Letters, 2004

A method for coating released polysilicon microstructures with thin, uniform and conformal coatin... more A method for coating released polysilicon microstructures with thin, uniform and conformal coatings of SiC derived from the single source precursor, 1,3-disilabutane (DSB) has been developed. This coating method has been successfully applied to micromechanical test devices which allow evaluation of friction and wear properties of the coating. Here, data on the coefficient of static friction of SiC coatings produced from DSB is presented. Also, a comparative wear study for devices which have been oxidized, treated with an anti-adhesion coating, and SiC coated is shown. Wear is examined by scanning electron microscopy (SEM) on devices which have been cycled repetitively under a nominal load. It is found that the application of a few nanometers-thin SiC coating provides exceptional wear resistance as well as significant reduction in friction on the microscale.

The Astrophysical Journal, 2001

A mixture of hydrides of buckminsterfullerene made by the interaction of atomic hydrogen with C 6... more A mixture of hydrides of buckminsterfullerene made by the interaction of atomic hydrogen with C 60 is investigated using high-resolution electron energy loss spectroscopy (HREELS) in ultrahigh vacuum. The energy-loss spectra of partially hydrogenated C 60 multilayers reveal vibrational features previously observed in infrared emission from interstellar and circumstellar dust clouds, including a broad loss envelope between 1150 and 1310 cm Ϫ1 , followed by a band at 1620 cm Ϫ1 in remarkable agreement with the canonical interstellar spacing of 300 cm Ϫ1. Additionally, a major C i H stretching band near 2900 cm Ϫ1 is observed and compared with the Galactic center absorption spectrum.

Physical Review Letters, 1993

Calculations are presented of the wetting properties of liquid hydrogen films on various substrat... more Calculations are presented of the wetting properties of liquid hydrogen films on various substrates. The well depth D for the adsorption potential is even smaller for alkali metal substrates than for noble gas surfaces. A simple model calculation leads to the prediction that wetting transitions should then occur at a temperature T about 20 K. Quartz microbalance data obtained for H2 on Rb are consistent with this result. Specifically, T is found to be 17.89 0.04 K. The wetting phase diagram is found to be in good agreement with thermodynamic expectations.

Nano Letters, 2007

We demonstrate a bottom-up/top-down combined method for the fabrication of horizontally suspended... more We demonstrate a bottom-up/top-down combined method for the fabrication of horizontally suspended, well-oriented and size-controlled Si nanowire arrays. Mechanical beamlike structures composed of multiple ordered arrays consecutively linked by transversal microspacers are obtained by this method. Such structures are used to investigate the mechanical elasticity of the nanowire arrays by atomic force microscopy. Our results point out important differences in the morphology and mechanical behavior of the fabricated nanowire-based structures with respect to equivalent bulk material structures.

Journal of the American Chemical Society, 2005

2012 IEEE 25th International Conference on Micro Electro Mechanical Systems (MEMS), 2012

Supercapacitors have attracted much attention for energy storage applications, owing to their lon... more Supercapacitors have attracted much attention for energy storage applications, owing to their long cycle life and high power densities. We report on the growth of silicon carbide (SiC) nanowires (NW) and the evaluation of their performance as electrode material for microsupercapacitors. Their specific capacitance has been studied as a function of their morphology (size, diameter) and the optimal growth conditions have lead to a capacitance comparable to the state of the art. They exhibit an exceptional stability, with a lifetime exceeding 10 6 charge/discharge cycles. An excellent thermal stability is expected for SiC NWs, opening the way to the fabrication of high temperature micro-supercapacitors.

Physical Review Letters, 2008

The early stages of epitaxial graphene layer growth on the Si-terminated 6H-SiC(0001) are investi... more The early stages of epitaxial graphene layer growth on the Si-terminated 6H-SiC(0001) are investigated by Auger electron spectroscopy (AES) and depolarized Raman spectroscopy. The selection of the depolarized component of the scattered light results in a significant increase in the CC bond signal over the second order SiC Raman signal, which allows to resolve submonolayer growth, including individual, localized C=C dimers in a diamond-like carbon matrix for AES C/Si ratio of ∼3, and a strained graphene layer with delocalized electrons and Dirac single-band dispersion for AES C/Si ratio >6. The linear strain, measured at room temperature, is found to be compressive, which can be attributed to the large difference between the coefficients of thermal expansion of graphene and SiC. The magnitude of the compressive strain can be varied by adjusting the growth time at fixed annealing temperature.

We formulate the quasielastic response of a non-relativistic many-body system at zero temperature... more We formulate the quasielastic response of a non-relativistic many-body system at zero temperature in terms of ground state density matrix elements and real time path integrals that embody the final state interactions. While the former provide the weight for a conventional Monte Carlo calculation, the latter require a more sophisticated treatment. We argue that the recently developed Stationary Phase Monte Carlo technique can be used to study the approach to "Y-scaling." We perform calculations for a particle in a potential well in one and three dimensions and compare them to the exact results available for these models. We then derive an eikonal approximation to the Path Integrals. This method is suitably generalized to treat strongly repulsive interactions, and allows comparison to Silver's theory of final state interactions in a straightforward way. We also give an exact prescription to calculate the scaling limit for potentials comprising a hard core. Finally, we st...