Ultralow dimensionality of 2D layers magnifies their sensitivity to adjacent charges enabling even postprocessing electric control of multifunctional structures. However, functionalizing 2D layers remains an important challenge for on-demand device-property exploitation. Here we report that an electrical and even fully optical way to control and write modifications to the magnetoresistive response of CVD-deposited graphene is achievable through the electrostatics of the photoferroelectric substrate. For electrical control, the ferroelectric polarization switch modifies graphene magnetoresistance by 67% due to a Fermi level shift with related modification in charge mobility. A similar function is also attained entirely by bandgap light due to the substrate photovoltaic effect. Moreover, an all-optical way to imprint and recover graphene magnetoresistance by light is reported as well as magnetic control of graphene transconductance. These findings extend photoferroelectric control in 2D structures to magnetic dimensions and advance wireless operation for sensors and field-effect transistors.

Interface-driven effects in ferroelectric van der Waals (vdW) heterostructures provide fresh opportunities in the search for alternative device architectures toward overcoming the von Neumann bottleneck. However, their implementation is still in its infancy, mostly by electrical control. It is of utmost interest to develop strategies for additional optical and multistate control in the quest for novel neuromorphic architectures. Here, we demonstrate the electrical and optical control of the ferroelectric polarization states of ferroelectric field effect transistors (FeFET). The FeFETs, fully made of ReS2/hBN/CuInP2S6 vdW materials, achieve an on/off ratio exceeding 107, a hysteresis memory window up to 7 V wide, and multiple remanent states with a lifetime exceeding 103 s. Moreover, the ferroelectric polarization of the CuInP2S6 (CIPS) layer can be controlled by photoexciting the vdW heterostructure. We perform wavelength-dependent studies, which allow for identifying two mechanisms at play in the optical control of the polarization: band-to-band photocarrier generation into the 2D semiconductor ReS2 and photovoltaic voltage into the 2D ferroelectric CIPS. Finally, heterosynaptic plasticity is demonstrated by operating our FeFET in three different synaptic modes: electrically stimulated, optically stimulated, and optically assisted synapse. Key synaptic functionalities are emulated including electrical long-term plasticity, optoelectrical plasticity, optical potentiation, and spike rate-dependent plasticity. The simulated artificial neural networks demonstrate an excellent accuracy level of 91% close to ideal-model synapses. These results provide a fresh background for future research on photoferroelectric vdW systems and put ferroelectric vdW heterostructures on the roadmap for the next neuromorphic computing architectures.

Graphene films are used to detect the presence and transition of spin crossover nanoparticle aggregates. Experiments performed far from the graphene neutrality point, combining impedance spectroscopy and Hall measurements, provide better insight into the mechanism for the change of impedance of the graphene layer in proximity with different states of the molecular structure. We observe that the change of spin state shifts the graphene Fermi level and its intrinsic resistance, with resulting positive insight into using this type of hybrid device for fast molecular electronics purposes.

Emerging reconfigurable devices are fast gaining popularity in the search for next-generation computing hardware, while ferroelectric engineering of the doping state in semiconductor materials has the potential to offer alternatives to traditional von-Neumann architecture. In this work, we combine these concepts and demonstrate the suitability of reconfigurable ferroelectric field-effect transistors (Re-FeFETs) for designing nonvolatile reconfigurable logic-in-memory circuits with multifunctional capabilities. Modulation of the energy landscape within a homojunction of a 2D tungsten diselenide (WSe2) layer is achieved by independently controlling two split-gate electrodes made of a ferroelectric 2D copper indium thiophosphate (CuInP2S6) layer. Controlling the state encoded in the program gate enables switching between p, n, and ambipolar FeFET operating modes. The transistors exhibit on-off ratios exceeding 106 and hysteresis windows of up to 10 V width. The homojunction can change from Ohmic-like to diode behavior with a large rectification ratio of 104. When programmed in the diode mode, the large built-in p-n junction electric field enables efficient separation of photogenerated carriers, making the device attractive for energy-harvesting applications. The implementation of the Re-FeFET for reconfigurable logic functions shows how a circuit can be reconfigured to emulate either polymorphic ferroelectric NAND/AND logic-in-memory or electronic XNOR logic with a long retention time exceeding 104 s. We also illustrate how a circuit design made of just two Re-FeFETs exhibits high logic expressivity with reconfigurability at runtime to implement several key nonvolatile 2-input logic functions. Moreover, the Re-FeFET circuit demonstrates high compactness, with an up to 80% reduction in transistor count compared to standard CMOS design. The 2D van de Waals Re-FeFET devices therefore exhibit promising potential for both More-than-Moore and beyond-Moore future of electronics, in particular for an energy-efficient implementation of in-memory computing and machine learning hardware, due to their multifunctionality and design compactness.

As global data generation continues to rise, there is an increasing demand for revolutionary in-memory computing methodologies and efficient machine learning solutions. Despite recent progress in electrical and electro-optical simulations of machine learning devices, the all-optical nonthermal function remains challenging, with single wavelength operation still elusive. Here we report on an optical and monochromatic way of neuromorphic signal processing for brain-inspired functions, eliminating the need for electrical pulses. Multilevel synaptic potentiation-depression cycles are successfully achieved optically by leveraging photovoltaic charge generation and polarization within the photoferroelectric substrate interfaced with the graphene sensor. Furthermore, the demonstrated low-power prototype device is able to reproduce exact signal profile of brain tissues yet with more than 2 orders of magnitude faster response. The reported properties should trigger all-optical and low power artificial neuromorphic development based on photoferroelectric structures.

We review the current understanding of the time scale and mechanisms associated with the change in spin state in transition metal-based spin crossover (SCO) molecular complexes. Most time resolved experiments, performed by optical techniques, rely on the intrinsic light-induced switching properties of this class of materials. The optically driven spin state transition can be mediated by a rich interplay of complexities including intermediate states in the spin state transition process, as well as intermolecular interactions, temperature, and strain. We emphasize here that the size reduction down to the nanoscale is essential for designing SCO systems that switch quickly as well as possibly retaining the memory of the light-driven state. We argue that SCO nano-sized systems are the key to device applications where the “write” speed is an important criterion.

The frequency dependence of dielectric constant for composites of polyaniline (PANI) and multi-walled carbon nanotube (MWCNT) with different degree of functionalization is studied at low temperature (down to 4.2 K) and magnetic field (up to 3 T) applied both in parallel and perpendicular direction of ac electric field. A relaxation phenomenon is observed in all the MWCNT/PANI composites by applying magnetic field in both the directions, below 103 Hz. However, PANI does not show any relaxation peak with applied magnetic field in either direction. The relaxation peak frequency does not depend on the strength of magnetic field but it varies with temperature and degree of functionalization of MWCNT in composites. This relaxation phenomenon occurs due to the inhomogeneity of the medium of two highly mismatched conductive materials at low temperatures. The results are explained in the light of Parish and Littlewood theory about magnetocapacitance in nonmagnetic composite.

The impedance, modulus spectroscopy and dielectric properties of polyaniline (PANI) and composite with multi-walled carbon nanotube (MWCNT) of different degrees of functionalization are studied from 300 to 4.2 K in the frequency range 40 Hz-5 MHz. At low temperature (T 100 K), grain and grain boundary relaxation peaks are observed in PANI. In composites, MWCNT forms a network between less and highly conducting grains, increases dc conductivity, and shows no relaxation peak at low temperature. The chemical functionalization of MWCNT increases the polar interaction between MWCNT and PANI, shifts the relaxation peaks towards lower frequency and slows the relaxation process. The relaxation process is manifested as non-Debye type and it tends towards Debye type with increasing temperature in functionalized muti-walled carbon nanotube (fMWCNT) PANI composites, and explained by Bergman's theory. The relaxation barrier energy at the interface of fMWCNT and PANI increases with increasing degree of functionalization which is the effect of enhanced interfacial (Maxwell Wagner-Sillars) polarization. In MWCNT composite, the real part of dielectric constant shows high negative value below 10 K at lower frequency (<950 Hz) and it becomes positive due to functionalization of MWCNT. An additional crossover from positive to negative occurs in both MWCNT and fMWCNT, PANI composite above temperature 60 K and frequency 500 Hz. This crossover shifts towards higher temperature and frequency with increasing degree of functionalization. The negative value and crossover of dielectric constant are explained by Drude-Sommerfeld model.

We report the ac conductivity measurement of polyaniline (PANI) composites with carbon nanotubes (CNT) and functionalized carbon nanotubes (fCNT) with different degrees of functionalization from 300 K-4.2 K in the frequency range 40 Hz-5 MHz. The ac conductivity of PANI follows the universal power law and the charge transport is dominated by small polaron tunneling. In the CNT/PANI composite, metallic behaviour emerges, and ac conductivity does not obey the power law which is explained by the Drude model. Due to the chemical functionalization of CNT, disordered semiconducting behaviour appears in the transport below 50 K and follows the Johnscher's power law. The enhancement of polaron formation due to the polar interaction between fCNT and PANI is reflected in the ac conductivity of the composites. The charge transport is manifested by single electron tunneling for a lower degree of fCNT/PANI (6 h) composite which changes to small polaron tunneling transport for a higher degree of fCNT/PANI (48 h) composite. The dc conductivity behaviour is explained by Mott's VRH model and the characteristic Mott temperature increases with an increasing degree of chemical functionalization.

The charge transport and magnetoresistance (MR) of polyaniline (PANI)/carbon nanotube (CNT) composite with increasing degree of chemical functionalized CNT have been studied from 300 to 4.2 K and magnetic field up to 5 T. The variation of resistivity with temperature is higher for fCNT/PANI compare to CNT/PANI composite and it increases with increasing degree of functionalization of CNT. The resistivity variation with temperature is explained by Mott's VRH and Efros–Shklovskii model. The chemical functionalization of CNT increases the electron- electron interaction in the composites. At low temperature (T=4.2 K) all the fCNT/PANI composites show positive MR although CNT/PANI shows negative. Lower degree of functionalized CNT/PANI composite (6 h) shows MR transition from positive to negative with increasing temperature (T ≥10 K) but higher degree of fCNT/PANI composites (48 h and 96 h) show no crossover in MR at measured temperature range. The transition of MR in fCNT/PANI composites is explained by NSS model. The increasing degree of functionalization increases the dipolar interaction between fCNT and PANI which leads to change in sign of tunneled electron scattering amplitude in charge transport that give rise to the MR transition in system.

Metacomposites with tunable negative permittivity find wide range of applications such as in novel electrical devices, flexible invisibility cloaks, stretchable sensors, etc. In the work reported here, the superior tunable negative permittivity of Polydimethylsiloxane (PDMS)-Graphite nanocomposites, over graphite and neat PDMS has been demonstrated. The ac conductivity and impedance of PDMS composites with varying graphite loading are studied as a function of frequency (100 Hz–100 MHz). Interesting phenomenon like capacitive-inductive transition and positive to negative switching of permittivity are observed and interrelationships between them is discussed here. The negative permittivity which is mainly due to the inductive conductive networks formed within the composites, is explained based on interband transition and Drude theory of metals. For 30 wt% composite maximum positive permittivity of 280.4 and maximum negative value of -29.87 is observed at 100 Hz and 3.35 × 107 Hz respectively. For higher graphite loading the permittivity switching frequency is found to shift to lower frequency region due to increased inductive behavior of the composites.

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is an important opto-electronic material, and its room temperature electrical conductivity can be prominently enhanced by chemical treatment; it is technologically significant to investigate its temperature and magnetic field dependent charge transport characteristics. Here, we analyzed the low temperature charge transport properties of pristine and acid-treated PEDOT:PSS thin films by studying the variation in resistance as a function of temperature and magnetic field (T ~300 to 4.2 K, H ~up to 5 T). The acid-treated sample exhibited much improved charge transport behavior at low T, with a resistivity ratio [ρ(4.2 K)/ρ(300 K)] of ~5.7 × 102, which is three orders of magnitude smaller than that of the pristine sample. Nevertheless, both pristine and acid-treated samples followed hopping conduction, obeying R (T) ∞ exp T -1/2. and R (T) ∞ exp T -1/3, respectively, in the low T regime. Furthermore, positive magnetoresistance of ~16% (at 4.2 K and 5 T) was displayed by acid-treated samples that could be due to the wave function shrinkage phenomenon.

In this report, we have investigated the magnetoresistance (MR) and Hall effect of the ferrimagnetic composites containing LaNiO3 and CoFe2O4 (CFO) (with CFO content 15% and 20%) which exhibit orbital two-channel Kondo (2CK) effect and therefore pronounced resistivity upturn at low temperature. Both composites manifest a negative to positive crossover in MR with increasing temperature. The MR is described by the Khosla and Fisher model of spin fluctuations scattering of conduction electrons and the two-band theory based on hybridized p-d sub-bands. The Hall resistivity of the composites consists of both ordinary and anomalous part. The negative sign of the ordinary Hall coefficient suggests electrons as the dominating charge carriers. The coefficient of anomalous Hall resistivity (RS) follows the scaling relation (RS = aρxx + bρ2 xx) with longitudinal resistivity (ρxx) at high temperature above the resistivity upturn. However, at low temperature RS shows non-monotonous behaviour and deviates from the scaling relation where orbital 2CK effect takes place. More detailed study below the resistivity upturn of the composite with 20% CFO reveals that this deviation occurs around the Kondo temperature. This breakdown of scaling relation around the Kondo temperature indicates the possible influence of orbital 2CK on the anomalous Hall effect.

We have studied the effect of chemical functionalization on charge transport property of multiwall carbon nanotube (MWNT) down to the temperature 4.2 K and magnetic field up to 5 T. The resistivity ratio (ρ r = [ρ(T)/ρ(100 K)]) increases with the increment of degree of functionalization at low temperature as the effect of inclusion of disorder in the samples. The variation of resistivity with temperature has been explained by 3D variable range hopping model and Coulomb gap Efros-Shklovskii model, and the result shows that higher degree of functionalization enhances the disorder induced electron-electron interaction. For all the measured temperature and degree of functionalization, MWNTs show negative magnetoresistance. Negative magnetoresistance has been explained by 3D weak localization model.

We report low temperature electrical resistivity and magnetoresistance (MR) measurements of conducting polyaniline (PANI) and multiwalled-carbon nanotube (MWCNT) composites. We have used an in-situ oxidative polymerization method to synthesize hydrochloric acid-doped PANI composites with MWCNT weight percentages of 0, 5, 10 and 15. The temperature dependence of resistivity is studied from room temperature to 4.2 K and analysed by a Mott variable range hopping (VRH) model. The resistivity increases from 1.1×10-3Ωm at 300 K to 65.75Ωm at 4.2 K, almost four orders of the magnitude change with temperature for pure PANI. Whereas the PANI composite with 15% MWCNTs shows less variation from 4.6×10-4 to 3.5×10-2Ωm. The huge change in resistivity is due to the localization of charge carriers in the presence of disorder. At 4.2 K MR shows transition from positive to negative with higher MWCNT loading. Samples with 5 and 10% MWCNTs show positive MR, whereas the 15% MWCNT loaded sample shows negative MR. The positive and negative MR are discussed in terms of the wave function shrinkage effect and quantum interference effect on VRH conduction.

We report the tuning from spin one channel to orbital two-channel Kondo (2CK) effect by varying CoFe2O4 (CFO) content in the composites with LaNiO3 (LNO) along with the presence of ferrimagnetism. Although there is no signature of resistivity upturn in the case of pure LNO, all the composites exhibit a distinct upturn in the temperature range of 30-80 K. For composites with lower percentage of CFO (10%), the electron spin plays the key role in the emergence of resistivity upturn which is affected by external magnetic field. On the other hand, when the CFO content is increased (15%), the upturn shows strong robustness against high magnetic field (14 T) and a crossover in temperature variation from to T 1/2 at the Kondo temperature, indicating the appearance of orbital 2CK effect. The orbital 2CK effect originates due to the scattering of conduction electrons from the structural two-level systems which is created at the interfaces between the two phases (LNO and CFO) of different crystal structures as well as inside the crystal planes. The specific heat data at low temperature (40 K), deviates from the usual linear temperature variation of the electronic contribution. With higher CFO content it shows more deviation which also indicates the increasing amount of two-level system. A negative magnetoresistance (MR) is observed at low temperature (<30 K) for composites containing both lower (10%) and higher percentage (15%) of CFO. We have analyzed the negative MR using Khosla and Fisher semi-empirical model based on spin dependent scattering of conduction electrons from localized spins.

Electrical resistivity and magnetoresistance(MR) in polyaniline(PANI) with carbon nanotube(CNT) and functionalized carbon nanotube(fCNT) composites have been studied for different weight percentages down to the temperature 4.2 K and up to magnetic field 5 T. Resistivity increases significantly in composite at low temperature due to functionalization of CNT compared to only CNT. Interestingly a transition from negative to positive magnetoresistance has been observed when the filler is changed from pure CNT to functionalized CNT after a certain percentage (10wt%) as the effect of more disorder in fCNT/PANI composite. This result depicts that the MR has strong dependency on disorder in the composite system. The transition of MR has been explained on the basis of polaron-bipolaron model. The long range Coulomb interaction between two polarons screened by disorder in the composite of fCNT/PANI, increases the effective on-site Coulomb repulsion energy to form bipolaron which leads to change the sign of MR from negative to positive.