Soft Carrier Multiplications by Hot Electrons in Graphene

By using Boltzmann formalism, we show that carrier multiplication by impact ionization can take place at relatively low electric fields during electronic transport in graphene. Because of the absence of energy gap, this effect is not characterized by a field threshold unlike in conventional semiconductors, but is a quadratic function of the electric field. We also show that the resulting current is an increasing function of the electronic temperature, but decreases with increasing carrier concentration

Quantum transport in graphene nanotransistors

Over the past decade, interest in using graphene in condensed-matter physics and materials science applications has exploded, owing to its unique electrical properties. Narrow strips of graphene, called graphene nanoribbons, also display exotic behavior. A nanoribbon’s edge geometry determines its electronic transport properties, and the rich behavior of conductance of nanoribbons in response to external potentials makes them ideal for use within transistors. In this thesis, we work towards creating an accurate model of graphene nanoribbon transistors, and we asses two possible applications which exploit their amazing potential. We begin by outlining the basic theoretical and computational framework for the model developed in this work. We then demonstrate the capability of graphene nanoribbon transistors, with nanopores, to electronically detect, characterize, and manipulate translocating DNA strands. Specifically, we explore the tunability of such devices, by examining the role of lattice geometry, such as a quantum point contact constriction, on their performance. We perform a demonstration of the ability to detect the passage of double and single-stranded DNA, through molecular dynamics simulations. The transistors presented are capable of sensing the helical shape of double-stranded DNA molecules, the unraveling of a DNA helix into a planar-zipper form, and the passage of individual nucleotides of a single strand of DNA through the nanopore. We outline a preliminary analysis on the proper design of a multilayer transistor stack to control both the electronic properties of the conducting membrane, as well as the motion of the DNA. Lastly, we present another type of nanoribbon device, an all-carbon spintronic transistor for use in cascaded logic circuits. A thorough analysis of the transport properties of zigzag nanoribbon transistors in magnetic fields, in addition to the design and construction of logic gate circuits containing these spintronic transistors, is presented

Impact Ionization and Carrier Multiplication in Graphene

We develop a model for carrier generation by impact ionization in graphene, which shows that this effect is non-negligible because of the vanishing energy gap, even for carrier transport in moderate electric fields. Our theory is applied to graphene field effect transistors for which we parametrize the carrier generation rate obtained previously with the Boltzmann formalism [A. Girdhar and J. Leburton, Appl. Phys. Lett. 99, 229903 (2011)] to include it in a self-consistent scheme and compute the transistor I-V characteristics. Our model shows that the drain current exhibits an "up-kick" at high drain biases, which is consistent with recent experimental data. We also show that carrier generation affects the electric field distribution along the transistor channel, which in turn reduces the carrier velocity

Biophysical mechanisms of single-cell interactions with microtopographical cues

Biophysical cues encoded in the extracellular matrix (ECM) are increasingly being explored to control cell behavior in tissue engineering applications. Recently, we showed that cell adhesion to microtopographical structures (“micropegs”) can suppress proliferation in a manner that may be blunted by inhibiting cellular contractility, suggesting that this effect is related to altered cell-scaffold mechanotransduction. We now directly investigate this possibility at the microscale through a combination of live-cell imaging, single-cell mechanics methods, and analysis of gene expression. Using time-lapse imaging, we show that when cells break adhesive contacts with micropegs, they form F-actin-filled tethers that extend and then rupture at a maximum, critical length that is greater than trailing-edge tethers observed on topographically flat substrates. This critical tether length depends on myosin activation, with inhibition of Rho-associated kinase abolishing topography-dependent differences in tether length. Using cellular de-adhesion and atomic force microscopy indentation measurements, we show that the micropegs enhance cell-scaffold adhesive interactions without changing whole-cell elasticity. Moreover, micropeg adhesion increases expression of specific mechanotransductive genes, including RhoA GTPase and myosin heavy chain II, and, in myoblasts, the functional marker connexin 43. Together, our data support a model in which microtopographical cues alter the local mechanical microenvironment of cells by modulating adhesion and adhesion-dependent mechanotransductive signaling

Cascaded Spintronic Logic With Low-Dimensional Carbon

Remarkable breakthroughs have established the functionality of graphene and carbon nanotube transistors as replacements to silicon in conventional computing structures, and numerous spintronic logic gates have been presented. However, an efficient cascaded logic structure that exploits electron spin has not yet been demonstrated. In this work, we introduce and analyse a cascaded spintronic computing system composed solely of low-dimensional carbon materials. We propose a spintronic switch based on the recent discovery of negative magnetoresistance in graphene nanoribbons, and demonstrate its feasibility through tight-binding calculations of the band structure. Covalently connected carbon nanotubes create magnetic fields through graphene nanoribbons, cascading logic gates through incoherent spintronic switching. The exceptional material properties of carbon materials permit Terahertz operation and two orders of magnitude decrease in power-delay product compared to cutting-edge microprocessors. We hope to inspire the fabrication of these cascaded logic circuits to stimulate a transformative generation of energy-efficient computing

Electrically Tunable Quenching of DNA Fluctuations in Biased Solid-State Nanopores

Nanopores offer sensors for a broad range of nanoscale materials, in particular ones of biological origin such as single- and double-stranded DNA or DNA–protein complexes. In order to increase single-molecule sensitivity, it is desirable to control biomolecule motion inside nanopores. In the present study, we investigate how in the case of a double-stranded DNA the single-molecule sensitivity can be improved through bias voltages. For this purpose we carry out molecular dynamics simulations of the DNA inside nanopores in an electrically biased metallic membrane. Stabilization of DNA, namely, a reduction in thermal fluctuations, is observed under positive bias voltages, while negative voltages bring about only negligible stabilization. For positive biases the stabilization arises from electrostatic attraction between the negatively charged DNA backbone and the positively charged pore surface. Simulations on a teardrop-shaped pore show a transverse shift of DNA position toward the sharp end of the pore under positive bias voltages, suggesting the possibility to control DNA alignment inside nanopores through geometry shaping. The present findings open a feasible and efficient route to reduce thermal noise and, in turn, enhance the signal-to-noise ratio in single-molecule nanopore sensing

Association of dietary intake with micronutrient deficiency in Indian school children: a cross-sectional study

Adequate nutrition is necessary during childhood and early adolescence for adequate growth and development. Hence, the objective of the study was to assess the association between dietary intake and blood levels of minerals (calcium, iron, zinc, and selenium) and vitamins (folate, vitamin B12, vitamin A, and vitamin D) in urban school going children aged 6–16 years in India, in a multicentric cross-sectional study. Participants were enrolled from randomly selected schools in ten cities. Three-day food intake data was collected using a 24-h dietary recall method. The intake was dichotomised into adequate and inadequate. Blood samples were collected to assess levels of micronutrients. From April 2019 to February 2020, 2428 participants (50⋅2 % females) were recruited from 60 schools. Inadequate intake for calcium was in 93⋅4 % (246⋅5 ± 149⋅4 mg), iron 86⋅5 % (7⋅6 ± 3⋅0 mg), zinc 84⋅0 % (3⋅9 ± 2⋅4 mg), selenium 30⋅2 % (11⋅3 ± 9⋅7 mcg), folate 73⋅8 % (93⋅6 ± 55⋅4 mcg), vitamin B12 94⋅4 % (0⋅2 ± 0⋅4 mcg), vitamin A 96⋅0 % (101⋅7 ± 94⋅1 mcg), and vitamin D 100⋅0 % (0⋅4 ± 0⋅6 mcg). Controlling for sex and socioeconomic status, the odds of biochemical deficiency with inadequate intake for iron [AOR = 1⋅37 (95 % CI 1⋅07–1⋅76)], zinc [AOR = 5⋅14 (95 % CI 2⋅24–11⋅78)], selenium [AOR = 3⋅63 (95 % CI 2⋅70–4⋅89)], folate [AOR = 1⋅59 (95 % CI 1⋅25–2⋅03)], and vitamin B12 [AOR = 1⋅62 (95 %CI 1⋅07–2⋅45)]. Since there is a significant association between the inadequate intake and biochemical deficiencies of iron, zinc, selenium, folate, and vitamin B12, regular surveillance for adequacy of micronutrient intake must be undertaken to identify children at risk of deficiency, for timely intervention