Modulation Of Charge Carrier Density Research Articles

Redox gating-induced modulation of charge carrier density and lattice expansion in LaNiO3 thin films

Redox gating involves the use of reversible redox functionalities combined with ionic electrolytes to substantially alter the charge carrier density in functional condensed materials. This modification leads to the emergence of physical properties not observed in the original material. In our study, we focus on redox gating applied to a LaNiO3 (001) film within a field-effect device and identify a critical gate voltage of 0.7 V. Hall measurements indicate that redox gating markedly increases the charge carrier density in LaNiO3, reaching over 1014 cm−2. This increase is primarily due to the injection of electrons into LaNiO3, which offsets the existing hole carriers. These adjustments in the carrier concentration result in reversible lattice expansion in LaNiO3 when gate voltages are below 0.7 V. This expansion correlates well with theoretical models that consider adjustments to the Ni–O bond length, influenced by oxygen ligand holes. Conversely, at gate voltages above 0.7 V, there are significant changes in resistivity, lattice structure, and Ni valence, stemming from the formation of oxygen vacancies in the LaNiO3 film.

The charge carrier density modulation in off-stoichiometric ZrNiSn leads to enhanced thermoelectric performance

The off-stoichiometry i.e., ZrNi1+δSn (0<δ<1), and carrier modulation are the two approaches that have separately been found effective mainly to reduce thermal conductivity and improve power factor. This work explores the combined effect of both of these approaches by tuning the carrier density by varying the Sb-concentration in off-stoichiometric ZrNi1.04Sn, which shifts the Fermi level towards the conduction band as confirmed by the band structure calculations. The maximum thermoelectric figure-of-merit ZT∼0.95 at ∼873 K was achieved in the optimum composition ZrNi1.04Sn0.975Sb0.025. For evaluating the device applicability of the synthesized materials, the cumulative temperature dependence (CTD) model was applied, predicting the single leg maximum efficiency ∼8.6% at ΔT ∼ 546 K. A series of calculations have been performed to predict electronic band structure and electronic transport properties to unearth the underlying physics. The microhardness and fracture toughness were determined to ensure that the enhanced thermoelectric performance is not deteriorating mechanical robustness.

Enhanced dielectric and optical performance of (Cu, Ag) co-doped ZnO nanostructures for electronic applications

ABSTRACT Pure and Cu/Ag co-doped ZnO nanoparticles were synthesised by the co-precipitation method. The phase and structural analysis of ZnO nanoparticles were examined by using XRD and Williamson-Hall plots. XRD spectra revealed that prepared samples exhibit hexagonal wurtzite structure of ZnO. The change in lattice parameters, bond length, dislocation density and strain indicates that Cu and Ag were successfully incorporated. UV vis studies indicated the red shift in the absorption band attributed to defects induced after dopant additions. The dielectric measurements showed that the dielectric constant was found to increase and the dielectric loss decreases with increase in dopant percentage. Additions of Cu and Ag in ZnO lattice may lead to supplement grain boundary defects which can increase polarisation. The relatively higher values of ac conductivity were also found due to modulation of charge carrier density after dopant additions which suggest that the diffusion controlled charge carrier hopping mechanism is involved.

Structural, electrical and optical properties of MgO-reduced graphene oxide nanocomposite for optoelectronic applications

MgO-reduced graphene oxide nanocomposites (NCs) were synthesized by a simple two-step chemical method. The microstructure, surface morphology, and composition of the prepared samples have been studied. X-ray diffractometer (XRD) analysis confirmed the crystalline cubic MgO nanoparticle and rGO sheets. Scanning electron microscope (SEM) showed the spherical MgO nanoparticles well dispersed over the graphene sheets. UV–visible spectroscopy analysis demonstrated that a red shift in the wavelength dependent absorbance curve. The band gap of the samples was found to be decreased with the increase of rGO content. The dielectric studies have been examined in the frequency range 500 Hz−5 MHz and found significant improvement in the dielectric constant, dielectric loss, and electric properties due to rGO addition.This is mainly attributed to the strong interfacial polarization (Maxwell–Wagner polarization) between MgO and rGO sheets. Further, the modulation of charge carrier density with rGO additions help to enhance the electrical conductivity of NCs and thus, encouraging to have wider application in electronic and energy technologies.

Flexible organic ion-gated transistors with low operating voltage and light-sensing application

Ion-gated transistors are attracting significant attention due to their low operating voltage (<1 V) and modulation of charge carrier density by ion-gating media. Here we report flexible organic ion-gated transistors based on the high mobility donor–acceptor conjugated copolymer poly[4-(4,4-dihexadecyl 4H-cyclopenta[1,2-b:5,4-b′]-dithiophen-2-yl)-alt[1,2,5]thiadiazolo[3,4c]pyridine](PCDTPT) and the ionic liquid [1-ethyl-3 methylimidazolium bis(trifluoromethylsulfonyl)imide] as the ion-gating medium. Electrical characteristics of devices made on both [rigid (SiO2/Si) and flexible (polyimide (PI))] substrates showed very similar values of hole mobility (∼1 cm2 V−1 s−1) and ON–OFF ratio (∼105). Flexible ion-gated transistors showed good mechanical stability at different bending curvature radii and under repetitive bending cycles. The mobility of flexible ion-gated transistors remained almost unchanged upon bending. After 1000 bending cycles the mobility decreased by 20% of its initial value. Flexible photodetectors based on PCDTPT ion-gated transistors showed photosensitivity and photoresponsivity values of 0.4 and 93 AW−1.

Large mobility modulation in ultrathin amorphous titanium oxide transistors

Recently, ultrathin metal-oxide thin film transistors (TFTs) have shown very high on-off ratio and ultra-sharp subthreshold swing, making them promising candidates for applications beyond conventional large-area electronics. While the on-off operation in typical TFTs results primarily from the modulation of charge carrier density by gate voltage, the high on-off ratio in ultrathin oxide TFTs can be associated with a large carrier mobility modulation, whose origin remains unknown. We investigate 3.5 nm-thick TiOx-based ultrathin TFTs exhibiting on-off ratio of ~106, predominantly driven by ~6-decade gate-induced mobility modulation. The power law behavior of the mobility features two regimes, with a very high exponent at low gate voltages, unprecedented for oxide TFTs. We find that this phenomenon is well explained by the presence of high-density tail states near the conduction band edge, which supports carrier transport via variable range hopping. The observed two-exponent regimes reflect the bi-exponential distribution of the density of band-tail states. This improved understanding would be significant in fabricating high-performance ultrathin oxide devices.

Nonreciprocal optical links based on time-modulated nanoantenna arrays: Full-duplex communication

Interference of transmitted and received signals hinders the simultaneous functionality of a conventional optical antenna as a transmitter and receiver which is required for full-duplex communication. The full-duplex communication schemes enabled by dense wavelength division multiplexed optical networks require distinct transmitter/receiver components operating at different wavelengths which increase the cost, complexity, and footprint of the physical layer. In this work, we demonstrate that an array of nanoantennas with leaky-wave architecture based on spatiotemporal modulation establishes nonreciprocal optical links which can reject the interference of transmitted and received signals by isolating the frequency of transmission and reception modes. For this purpose, we integrate indium-tin oxide into plasmonic nanodipoles which allows for realization of time-modulated nanoantennas in near-infrared frequency regime through electrical modulation of charge carrier density with radio-frequency signals. The radiation characteristics of individual nanoantennas and modal properties of nanoantenna arrays are rigorously studied through linking of charge transport and electromagnetic models. To this end, we extend the formulation of discrete dipole approximation as the standard modeling tool for electromagnetic scattering from nanoantenna arrays to treat realistic time-modulated structures with drastically different timescales between optical and modulation frequencies. The operation of spatiotemporally modulated array antennas in transmission and reception modes is investigated. Moreover, electrical beam-scanning functionality and dependence of antenna characteristics to modulation parameters and wavelength are demonstrated. It is rigorously established that such array antennas can operate as full transceivers by separating the transmitted and received signals propagating along the same direction through down conversion and up conversion of the frequency. Our results provide a route toward realization of optical antenna systems capable of full-duplex communication and real-time beam scanning which can increase the capacity and decrease the complexity of optical networks.

Enhanced Gas-Sensing Performance of GO/TiO₂ Composite by Photocatalysis.

Few studies have investigated the gas-sensing properties of graphene oxide/titanium dioxide (GO/TiO2) composite combined with photocatalytic effect. Room temperature gas-sensing properties of the GO/TiO2 composite were investigated towards various reducing gases. The composite sensor showed an enhanced gas response and a faster recovery time than a pure GO sensor due to the synergistic effect of the hybridization, such as creation of a hetero-junction at the interface and modulation of charge carrier density. However, the issue of long-term stability at room temperature still remains unsolved even after construction of a composite structure. To address this issue, the surface and hetero-junction of the GO/TiO2 composite were engineered via a UV process. A photocatalytic effect of TiO2 induced the reduction of the GO phase in the composite solution. The comparison of gas-sensing properties before and after the UV process clearly showed the transition from n-type to p-type gas-sensing behavior toward reducing gases. This transition revealed that the dominant sensing material is GO, and TiO2 enhanced the gas reaction by providing more reactive sites. With a UV-treated composite sensor, the function of identifying target gas was maintained over a one-month period, showing strong resistance to humidity.

Doping-Induced Tunable Wettability and Adhesion of Graphene.

We report that substrate doping-induced charge carrier density modulation leads to the tunable wettability and adhesion of graphene. Graphene's water contact angle changes by as much as 13° as a result of a 300 meV change in doping level. Upon either n- or p-type doping with subsurface polyelectrolytes, graphene exhibits increased hydrophilicity. Adhesion force measurements using a hydrophobic self-assembled monolayer-coated atomic force microscopy probe reveal enhanced attraction toward undoped graphene, consistent with wettability modulation. This doping-induced wettability modulation is also achieved via a lateral metal-graphene heterojunction or subsurface metal doping. Combined first-principles and atomistic calculations show that doping modulates the binding energy between water and graphene and thus increases its hydrophilicity. Our study suggests that the doping-induced modulation of the charge carrier density in graphene influences its wettability and adhesion [corrected]. This opens up unique and new opportunities for the tunable wettability and adhesion of graphene for advanced coating materials and transducers.

Protonic/electronic hybrid oxide transistor gated by chitosan and its full-swing low voltage inverter applications

Modulation of charge carrier density in condensed materials based on ionic/electronic interaction has attracted much attention. Here, protonic/electronic hybrid indium-zinc-oxide (IZO) transistors gated by chitosan based electrolyte were obtained. The chitosan-based electrolyte illustrates a high proton conductivity and an extremely strong proton gating behavior. The transistor illustrates good electrical performances at a low operating voltage of ∼1.0 V such as on/off ratio of ∼3 × 107, subthreshold swing of ∼65 mV/dec, threshold voltage of ∼0.3 V, and mobility of ∼7 cm2/V s. Good positive gate bias stress stabilities are obtained. Furthermore, a low voltage driven resistor-loaded inverter was built by using an IZO transistor in series with a load resistor, exhibiting a linear relationship between the voltage gain and the supplied voltage. The inverter is also used for decreasing noises of input signals. The protonic/electronic hybrid IZO transistors have potential applications in biochemical sensors and portable electronics.

Substrate Level Control of the Local Doping in Graphene

Graphene exfoliated onto muscovite mica is studied using ultrahigh vacuum scanning tunneling microscopy (UHV-STM) techniques. Mica provides an interesting dielectric substrate interface to measure the properties of graphene due to the ultraflat nature of a cleaved mica surface and the surface electric dipoles it possesses. Flat regions of the mica surface show some surface modulation of the graphene topography (24 pm) due to topographic modulation of the mica surface and full conformation of the graphene to that surface. In addition to these ultraflat regions, plateaus of varying size having been found. A comparison of topographic images and STS measurements show that these plateaus are of two types: one with characteristics of water monolayer formation between the graphene and mica, and the other arising from potassium ions trapped at the interfacial region. Immediately above the water induced plateaus, graphene is insulated from charge doping, while p-type doping is observed in areas adjacent to these water nucleation points. However, above and in the neighborhood of interfacial potassium ions, only n-type doping is observed. Graphene regions above the potassium ions are more strongly n-doped than regions adjacent to these alkali atom plateaus. Furthermore, a direct correlation of these Fermi level shifts with topographic features is seen without the random charge carrier density modulation observed in other dielectric substrates. This suggests a possible route to nanoscopic control of the local electron and hole doping in graphene via specific substrate architecture.

Contactless Characterization of Electronic Properties of Nanomaterials Using Dielectric Force Microscopy

Characterization of electronic properties of nanomaterials usually involves fabricating field effect transistors and deriving materials properties from device performance measurements. The difficulty in fabricating electrical contacts to extremely small-sized nanomaterials as well as the intrinsic heterogeneity of nanomaterials makes it a challenging task to measure the electronic properties of large numbers of individual nanomaterials. Here, we utilize a scanning probe technique, the dielectric force microscopy (DFM) to address the challenges. The DFM technique measures the low frequency dielectric response of nanomaterials, which is intrinsically related to their electrical conductivity. The incorporation of a gate bias voltage in DFM measurements allows for charge carrier density modulation, which is exploited to determine the carrier type in nanomaterials such as semiconducting single-walled carbon nanotubes (SWNTs) and ZnO nanowires (ZnO NWs). This technique avoids the need of electrical contacts and inherits the spatial mapping capability of scanning probe microscopy, as manifested in the imaging of intratube metallic/semiconducting junctions in SWNTs. We expect the DFM technique to find broad applications in the characterization of various nanoelectonic materials and nanodevices.

LVI detection on passive structure in advance CMOS technology: New opportunities for device analysis

For very deep submicron technologies, 45nm and below, Photoemission microscopy suffers from decreasing signal strength due to lower voltages. Laser Voltage Imaging (LVI) technique, introduced in 2009, allows mapping frequency through the backside of integrated circuit. For 1340nm laser wavelength, the measured reflected signal is related to charge carrier density modulation. This signal is measured on active areas at transistor level. In this paper we discuss about the LVI (Laser Voltage Imaging) signal observed on passive structures such as copper and polysilicon resistors. We describe the way to use the LVI technique, usually dedicated to frequency mapping of digital active parts, for the location of resistive leakage. The origin of this signal is investigated including charge carrier density variations and thermo reflectance effect. Experimental results on 45nm technology are presented. We show that the ability to perform ‘LVI’ measurements on passive structures “open the door” for the characterization of deep submicron devices.