Publications

The electrostatic properties of nanowire field effect transistors with triangular cross sections were investigated. The Poisson equation was solved for these structures; furthermore, two properties of the nanowire field effect transistors, the gate capacitance and current versus gate voltage, were calculated. The simulation results yielded the type, mobility, and concentration of the carriers, as well as the Ohmic contact resistance of the wire transistor. We examined how wire capacitance depends on various parameters: wire diameter, gate oxide thickness, charge density, and shape. It is shown that the capacitance of a triangular nanowire is less than that of a cylindrical nanowire of the same size, which could be significant in structures with thin gate oxides. The simulation results were compared with the previously reported experimental data on GaN nanowires.

Solution-processed organic photovoltaic devices containing p-type and non-fullerene n-type small molecules obtain power conversion efficiencies as high as 2.4%. The optoelectronic properties of the n-type material BT(TTI-n12)2 allow these devices to display high open-circuit voltages (>0.85 V) and generate significant charge carriers through hole transfer in addition to the electron-transfer pathway, which is common in fullerene-based devices. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

In this work, we report lateral heterojunction formation in as-exfoliated MoS2 flakes by thickness modulation. Kelvin probe force microscopy is used to map the surface potential at the monolayermultilayer heterojunction, and consequently the conduction band offset is extracted. Scanning photocurrent microscopy is performed to investigate the spatial photocurrent response along the length of the device including the source and the drain contacts as well as the monolayermultilayer junction. The peak photocurrent is measured at the monolayer-multilayer interface, which is attributed to the formation of a type-I heterojunction. The work presents experimental and theoretical understanding of the band alignment and photoresponse of thickness modulated MoS2 junctions with important implications for exploring novel optoelectronic devices. Semiconducting transition metal dichalcogenides (TMDCs) with a layered crystal structure exhibit unique electrical 1,2 and optical properties 3–5 . TMDCs provide opportunities in exploring new device concepts given their atomic level flatness, and ability to form van der Waals (vdW) heterostructures with strong interlayer coupling 6–8 . For instance, vdW heterobilayers of MoS2/WSe2 have been recently reported to exhibit spatially direct light absorption but spatially indirect light emission, representing a highly intriguing material property 9,10 . Here, we explore the optoelectronic properties of lateral “hetero”-junctions formed on a single crystal of MoS2 of varying thickness (i.e., number of layers). As a result of the quantum confinement effect 11 , when the thickness of a MoS2 crystal is scaled down to a monolayer the optical band gap increases from 1.29 eV (indirect) to 1.85 eV (direct) 12,13 . The change in the band structure and the electron affinity of MoS 2 with layer number opens up the path to the formation of atomically sharp heterostructures, not by changing composition but rather by changing layer thickness 14 . We experimentally examine the surface potential of this thickness modulated heterojunction by using Kelvin probe force microscopy (KPFM). We further use scanning photocurrent microscopy (SPCM) to probe the photoresponse of the junction. A large photocurrent response is observed at the monolayer/ multilayer junction interface which confirms the presence of a strong built-in electric field at the inter face. Device modeling is used in parallel to experiments to understand the underlying mechanism of the observed photocurrents and the band-alignments at the junction interface, suggesting the formation of a type-I heterojunction. SPCM has been previously used to study the photoresponse of metal contacted MoS2 transistors, where the channel thickness for MoS2 was uniform throughout the device 15,16 . The results have shown that the photoresponse is primarily driven by the metal/MoS2 Schottky contacts and photothermoelectric effect 16 . In distinct contrast to previous studies, we observe that the peak photoresponse is spatially located at the MoS2 monolayer/multilayer junction for our lateral heterojunctions and not at the metal contacts.

<p>Supplementary Table 4. The Adjusted Associations of Body Composition Measurements (Per SD Increase) with Reduced RDI Counting for Each Chemotherapy Agent</p>

Memristors, which are the fourth kind of passive element, along with resistors, inductances and capacitors, were first explicitly described in [1]. Their main characteristics are: (i) if there is no voltage across its terminals, there is no current; and (ii) the I-V curve shows a hysteresis that depends on the frequency of the forcing signal. Equations (1)-(3) describe the most general class of memristors [2], including a “hidden” set of controlling variables (for instance, temperature). v(t)=R(Q,X)i(t) (1) dX/dt=g(Q,i,X) (2) dQ/dt=i(t) (3) v(t) and i(t) are respectively the voltage and the current in the device, R(Q,X) is the memristance of the device, X is the set of controlling variables. Equation (3) is the general relation between charge and current, and is used without change along this paper. A special case of (2) is the POP equation (power-off plot), which is the case of no external forcing stimuli. For a memristor to be used as a non-volatile memory (as ReRAMs or PCM are), the POP equation (2) must equal zero for i=0 and any Q and X. Otherwise, X and, thus, R(Q,X) will change with time. In semiconductor devices, many phenomena involving some charge trapping and releasing are easily described with equations (1)-(3). Let’s analyze two different cases where this formalism is helpful: contact effects and hysteresis effects. Contact effects: Usually, contact effects are modeled using some passive elements, like resistors or diodes. There are other approaches, however, that model them in a more physical way [3] which can be very easily reformulated using memristor formalism as in equations (4)-(5). R(Q,X)=M·(Vgs-Vth)^n (4) dVth/dt= a·i(t)-b·(Vth-Vth0) (5) There, a and b are constants related to the probability of trapping and releasing charge at the interface, M and n are constants of the material, and Vth0 is the unperturbed threshold voltage. The physical mechanism behind the contact effect is a change in the effective barrier of the metal-semiconductor interface due to the charge trapped in there. Thus, memristors can be used as a discrete contact effect element, and implemented as a parasitic element. Hysteresis effects: Another effect observed in transistors is hysteresis, defined as a difference of the transistor characteristics between two consecutive measurements for identical inputs. As shown in [4], hysteresis can be explained as a change in the threshold voltage due to the accumulation of charge in the gate interface. This change can be described as in equations (6)-(7): i(t)=f(Q,Vth,Vds,Vgs) (6) dVth/dt= a·i(t)-b·(Vth-Vth0) (7) where a and b and Vth0 have the same meaning as in (4)-(5), f is the model for the current, Vgs is the gate-source voltage, and Vds is the drain-source voltage. Notice that the POP equation (7) doesn’t equal zero at i=0, so the system will relax back to its ground state Vth=Vth0. Figure 1 shows the effect of applying consecutive up and down Vgs ramps for a fixed Vds on a device modeled using this schema. As can be seen, the effect of hysteresis is higher at high currents, an leads to a change in the effective threshold voltage, as observed in real devices. Another mechanism producing hysteresis is the change in the mobility due to the filling of traps in the semiconductor[5]. In the VRH model [6], this leads to a change of the mobility. This effect seems to be smaller than the contribution of the change in threshold voltage in organic transistors, while it can be important in organic solar cells. Using the generic mobility model with two parameters [6], we obtain equations (8)-(10): i(t)=f(Q,Vaa,Ta,Vds,Vgs) (8) dVaa/dt= a1·i(t)- a2·(Vaa-Vaa0) (9) dTa/dt= b1·i(t)- b2·(Ta-Ta0) (10) where Vaa and Ta are the parameters of the mobility model, related to the trap distribution and the rest of the semiconductor characteristics [6]. Notice that equations (9)-(10) are a linear approximation by neglecting higher order non-linear terms. References [1] L. O. Chua, “Memristor-the missing circuit element,” Circuit Theory, IEEE Transactions, vol. 18, no. 5, pp. 507–519, 1971. [2] F. Corinto, et al, “A theoretical approach to memristor devices,” IEEE J. Emerg. Sel. Topics Circuits Syst., vol. 5, no. 2, pp. 123–132, 2015 [3] P. López-Varo, et al. “Charge density at the contacts of symmetric and asymmetric organic diodes“, Organic Electronics, 35, 74-86, 2016 [4] T. A. Karatsori, et al.,“Study of Hot Carrier-Induced Traps in Nanoscale UTBB FD-SOI MOSFETs by Low-Frequency Noise Measurements”,IEEE TED, 63(8), 2016 [5] R. Picos, et al. "Charge Dependent Mobility Memristormodel" Proc. of CDE’2015 [6] M. Estrada, et al. “Modeling the behavior of charge carrier mobility with temperature in thin-film polymeric transistors”, Microelectronic Engineering 87 (12), 2565-2570, 2010 Figure 1