Change In Carrier Density Research Articles

(Invited) Tunable Infrared Optoelectronics Achieved by the Reversible Intercalation of Ionic Liquid Anions in Multilayer Graphene in the Mid-Infrared Wavelength Range

We report the use of multilayer graphene (MLG) grown by chemical vapor deposition (CVD) approximately 75 layers (26 nm) thick configured in a simple electrochemical device. Applying a voltage of 3–4 V drives the intercalation of anion [TFSI-] (ion liquid diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide [DEME][TFSI]]) resulting in the reversible modulation of the properties of this optically dense material. Upon intercalation, we observe an abrupt shift of 35 cm−1 in the G band Raman mode, an abrupt increase in FTIR reflectance over the wavelength range from 1.67 to 5 μm (2000–6000 cm−1 ), and an abrupt increase in luminescent background observed in the Raman spectra of graphene. All of these abrupt changes in the optical properties of this material arise from the intercalation of the TFSI− ion and the associated change in the free carrier density (Δn = 1021 cm−3). Suppression of the 2D band Raman mode observed around 3 V corresponds to Pauli blocking of the double resonance Raman process and indicates a modulation of the Fermi energy of ΔEF= 1.1 eV. The thermal emissivity of the MLG device is also substantially modulated over the range from 7 – 14 µm. Using this device, have demonstrated free-space optical communication that utilizes ambient Planck radiation from a warm body and modulates the emitted intensity. We present an experimental demonstration achieving error-free communications at 100 bits per second over laboratory scales. We have also developed an AC Raman technique for measuring the Raman spectra during the intercalation−deintercalation half cycles in a multilayer graphene (MLG) device operating at 0.2−10 Hz. Raman spectra taken just 200 ms apart show the emergence and disappearance of the intercalated G band mode at around 1610 cm−1 . By integration of over hundreds of cycles, a significant Raman signal can be obtained. The intercalation/ deintercalation is also monitored with thermal imaging via voltage-induced changes in the carrier density, complex dielectric function ε(ω), and thermal emissivity of the device. “Dynamic Study of Intercalation/Deintercalation of Ionic Liquids in Multilayer Graphene Using an Alternating Current Raman Spectroscopy Technique” Zhi Cai, Haley Weinstein, Indu Aravind, Ruoxi Li, Sizhe Weng, Boxin Zhang, Jonathan L. Habif, and Stephen B. Cronin. Journal of Physical Chemistry Letters, 14, 7223 (2023).“Gate-tunable modulation of the optical properties of multilayer graphene by the reversible intercalation of ionic liquid anions” Zhi Cai, Indu Aravind, Haley Weinstein, Ruoxi Li, Jiangbin Wu, Han Wang, Jonathan Habif, and Stephen Cronin. Journal of Applied Physics. 132, 095102 (2022).“Harvesting Planck radiation for free-space optical communications in the long-wave infrared band“ Haley A. Weinstein, Zhi Cai, Stephen B. Cronin, and Jonathan L. Habif, Optics Letters. 47, 6225-6228 (2022). Figure 1

Non-equilibrium transport in polymer mixed ionic-electronic conductors at ultrahigh charge densities.

Conducting polymers are mixed ionic-electronic conductors that are emerging candidates for neuromorphic computing, bioelectronics and thermoelectrics. However, fundamental aspects of their many-body correlated electron-ion transport physics remain poorly understood. Here we show that in p-type organic electrochemical transistors it is possible to remove all of the electrons from the valence band and even access deeper bands without degradation. By adding a second, field-effect gate electrode, additional electrons or holes can be injected at set doping states. Under conditions where the counterions are unable to equilibrate in response to field-induced changes in the electronic carrier density, we observe surprising, non-equilibrium transport signatures that provide unique insights into the interaction-driven formation of a frozen, soft Coulomb gap in the density of states. Our work identifies new strategies for substantially enhancing the transport properties of conducting polymers by exploiting non-equilibrium states in the coupled system of electronic charges and counterions.

Direct imaging of the magnetoelectric coupling in multiferroic BaTiO3/La0.9Ba0.1MnO3

We report the direct imaging of the magnetic response of a 4.8 nm La0.9Ba0.1MnO3 film to the voltage applied across a 5 nm BaTiO3 film in a BaTiO3/La0.9Ba0.1MnO3 multiferroic heterostructure using x-ray photoemission electron microscopy (XPEEM). Specifically, we have written square ferroelectric domains on the BaTiO3 layer with an atomic force microscope in contact mode and imaged the corresponding magnetic contrast through the x-ray circular dichroic effect at the Mn L-edge with high spatial lateral resolution using XPEEM. We find a sudden decrease in the magnetic contrast for positive writing voltages above +6 V associated with the switching of the ferroelectric polarization of the BaTiO3, consistent with the presence of a magnetoelectric effect through changes in the hole carrier density at the BaTiO3/La0.9Ba0.1MnO3 interface. Temperature-dependent measurements show a decrease in the Curie temperature and magnetic moment in the areas where a positive voltage above +6 V was applied, corresponding to the hole depletion state and suggesting the onset of a spin-canted state of bulk La0.9Ba0.1MnO3. Our results are the first direct imaging of magnetoelectric coupling in such multiferroic heterostructure.

Optical spectroscopic detection of Schottky barrier height at a two-dimensional transition-metal dichalcogenide/metal interface.

Atomically thin two-dimensional transition-metal dichalcogenides (2D-TMDs) have emerged as semiconductors for next-generation nanoelectronics. As 2D-TMD-based devices typically utilize metals as the contacts, it is crucial to understand the properties of the 2D-TMD/metal interface, including the characteristics of the Schottky barriers formed at the semiconductor-metal junction. Conventional methods for investigating the Schottky barrier height (SBH) at these interfaces predominantly rely on contact-based electrical measurements with complex gating structures. In this study, we introduce an all-optical approach for non-contact measurement of the SBH, utilizing high-quality WS2/Au heterostructures as a model system. Our approach employs a below-bandgap pump to excite hot carriers from the gold into WS2 with varying thicknesses. By monitoring the resultant carrier density changes within the WS2 layers with a broadband probe, we traced the dynamics and magnitude of charge transfer across the interface. A systematic sweep of the pump wavelength enables us to determine the SBH values and unveil an inverse relationship between the SBH and the thickness of the WS2 layers. First-principles calculations reveal the correlation between the probability of injection and the density of states near the conduction band minimum of WS2. The versatile optical methodology for probing TMD/metal interfaces can shed light on the intricate charge transfer characteristics within various 2D heterostructures, facilitating the development of more efficient and scalable nano-electronic and optoelectronic technologies.

Making and Breaking of Exciton Cooling Bottlenecks in Halide Perovskite Nanocrystals.

Harnessing quantum confinement (QC) effects in semiconductors to retard hot carrier cooling (HCC) is an attractive approach for enabling efficient hot carrier extraction to overcome the Shockley-Queisser limit. However, there is a debate about whether halide perovskite nanocrystals (PNCs) can effectively exploit these effects. To address this, we utilized pump-probe and multipulse pump-push-probe spectroscopy to investigate HCC behavior in PNCs of varying sizes and cation compositions. Our results validate the presence of an intrinsic phonon bottleneck with clear manifestations of QC effects in small CsPbBr3 PNCs exhibiting slower HCC rates compared to those of larger PNCs. However, the replacement of inorganic Cs+ with organic cations suppresses this intrinsic bottleneck. Furthermore, PNCs exhibit distinct size-dependent HCC behavior in response to changes in the cold carrier densities. We attribute this to the enhanced exciton-exciton interactions in strongly confined PNCs that facilitate Auger heating. Importantly, our findings dispel the existing controversy and provide valuable insights into design principles for engineering QC effects in PNC hot carrier applications.

Effects of carrier density and interactions on pairing symmetry in a t2g model

By utilizing the fluctuation exchange approximation method, we perform a study on the superconducting pairing symmetry in a t2g three-orbital model on the square lattice. Although the tight-binding parameters of the model are based on Sr2RuO4, we have systematically studied the evolution of superconducting pairing symmetry with the carrier density and interactions, making our findings relevant to a broader range of material systems. Under a moderate Hund’s coupling, we find that spin fluctuations dominate the superconducting pairing, leading to a prevalent spin-singlet pairing with a d x 2–y 2 -wave symmetry for the carrier density within the range of n = 1.5–4 per site. By reducing the Hund’s coupling, the charge fluctuations are enhanced and play a crucial role in determining the pairing symmetry, leading to a transition of the pairing symmetry from the spin-singlet d x 2–y 2 -wave to the spin-triplet p-wave. Furthermore, we find that the superconducting pairings are orbital dependent. As the carrier density changes from n = 4 to n = 1.5, the active orbitals for superconducting pairing shift from the quasi-two-dimensional orbital d xy to the quasi-one-dimensional orbitals d xz and d yz .

Long-term stability of molecular doped epigraphene quantum Hall standards: single elements and large arrays (R K/236 ≈ 109 Ω)

In this work we investigate the long-term stability of epitaxial graphene (epigraphene) quantum Hall resistance standards, including single devices and an array device composed of 236 elements providing R K/236 ≈ 109 Ω, with R K the von Klitzing constant. All devices utilize the established technique of chemical doping via molecular dopants to achieve homogenous doping and control over carrier density. However, optimal storage conditions and the long-term stability of molecular dopants for metrological applications have not been widely studied. In this work we aim to identify simple storage techniques that use readily available and cost-effective materials which provide long-term stability for devices without the need for advanced laboratory equipment. The devices are stored in glass bottles with four different environments: ambient, oxygen absorber, silica gel desiccant, and oxygen absorber/desiccant mixture. We have tracked the carrier densities, mobilities, and quantization accuracies of eight different epigraphene quantum Hall chips for over two years. We observe the highest stability (i.e. lowest change in carrier density) for samples stored in oxygen absorber/desiccant mixture, with a relative change in carrier density below 0.01% per day and no discernable degradation of quantization accuracy at the part-per-billion level. This storage technique yields a comparable stability to the currently established best storage method of inert nitrogen atmosphere, but it is much easier to realize in practice. It is possible to further optimize the mixture of oxygen absorber/desiccant for even greater stability performance in the future. We foresee that this technique can allow for simple and stable long-term storage of polymer-encapsulated molecular doped epigraphene quantum Hall standards, removing another barrier for their wide-spread use in practical metrology.

Fermi-level dependence of magnetism and magnetotransport in the magnetic topological insulators Bi2Te3 and BiSbTe3 containing self-organized MnBi2Te4 septuple layers

The magnetic coupling mechanisms underlying ferromagnetism and magnetotransport in magnetically doped topological insulators (TIs) have been a central issue to gain controlled access to the magnetotopological phenomena such as the quantum anomalous Hall effect (AHE) and the topological axion insulating state. Here, we focus on the role of bulk carriers in magnetism of the family of magnetic TIs, in which the host material is either ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ or $\mathrm{BiSb}{\mathrm{Te}}_{3}$, containing Mn self-organized in $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$ septuple layers. We tune the Fermi level using the electron irradiation technique and study how magnetic properties vary through the change in carrier density; the role of the irradiation defects is also discussed. Ferromagnetic resonance spectroscopy and magnetotransport measurements show no effect of the Fermi-level position on the magnetic anisotropy field and the Curie temperature, respectively, excluding bulk magnetism based on a carrier-mediated process. Furthermore, the magnetotransport measurements show that the AHE is dominated by the intrinsic and dissipationless Berry-phase-driven mechanism, with the Hall resistivity enhanced near the bottom/top of the conduction/valence band, due to the Berry curvature which is concentrated near the avoided band crossings. These results demonstrate that the AHE can be effectively managed, maximized, or turned off by adjusting the Fermi level.

Broad Electronic Modulation of Two-Dimensional Metal-Organic Frameworks over Four Distinct Redox States.

Two-dimensional (2D) inorganic materials have emerged as exciting platforms for (opto)electronic, thermoelectric, magnetic, and energy storage applications. However, electronic redox tuning of these materials can be difficult. Instead, 2D metal-organic frameworks (MOFs) offer the possibility of electronic tuning through stoichiometric redox changes, with several examples featuring one to two redox events per formula unit. Here, we demonstrate that this principle can be extended over a far greater span with the isolation of four discrete redox states in the 2D MOFs LixFe3(THT)2 (x = 0-3, THT = triphenylenehexathiol). This redox modulation results in 10,000-fold greater conductivity, p- to n-type carrier switching, and modulation of antiferromagnetic coupling. Physical characterization suggests that changes in carrier density drive these trends with relatively constant charge transport activation energies and mobilities. This series illustrates that 2D MOFs are uniquely redox flexible, making them an ideal materials platform for tunable and switchable applications.

Engineering Magnetism and Superconductivity in van der Waals Materials via Organic‐Ion Intercalation

AbstractIntercalation is the insertion of guest species between the planes of a host van der Waals layered crystal. The process is accompanied by a significant change of the charge carrier density and by the expansion of the interlayer distance, overall leading to a modification of the electronic band structure of the layered material. This perspective focuses on the possibilities offered by the intercalation of organic ions toward finely tuning the physical properties of van der Waals materials, in particular their magnetism and superconductivity. How the intercalation of organic ions offers several advantages over conventional guest species such as alkali metals is highlighted, since a careful choice of the molecular intercalant opens the possibility to tailor the interlayer distance and the charge carrier density. Moreover, specific properties of the molecular guest can be transferred to the host material, as recently demonstrated by the intercalation of thermo‐responsive and chiral molecules. It is anticipated that other functional organic ions can be incorporated in van der Waals materials to provide additional optical and magnetic capabilities, with the potential to enable an optical control of magnetism and superconductivity.

Band Structure and Exciton Dynamics in Quasi‐2D Dodecylammonium Halide Perovskites

AbstractFemtosecond transient absorption spectroscopy (FTAS) is an important tool to investigate the physics of halide perovskites having different dimensionality, morphology, and architectures, giving insights into the electronic and excitonic optical transitions. Here, FTAS on monolayer (n = 1) and multilayer (n = 2, 3) quasi‐2D perovskites in the Ruddlesden–Popper phase: DA2MAn‐1PbnI3n+1 (DAMAPI) is presented, with the dodecylammonium (DA = CH3‐(CH2)11‐NH3+) as the spacer and methylammonium (MA = CH3NH3+) as the organic cation for samples with n > 1. The measurements, performed at 77 K and room temperature using several pump energies and excitation densities, allow the observation of different absorption bleaching energies. Those energies are compared with the results of first‐principles theoretical simulations based on density functional theory, the GW method, and the Bethe–Salpeter equation and assigned to transitions involving excitons with principal quantum numbers 1s and 2s. The temporal analysis of the absorption bleaching indicates the exciton–exciton annihilation as the main relaxation mechanism in the first picoseconds after excitation, while exciton radiative recombination is observed at longer time delays (>100 ps). Therefore, FTAS allows the study of the carrier dynamics and, given its high sensitivity to carrier density changes, the observation of spectral features not observable with steady‐state measurements.

Controllable Phase Transition Properties in VO2 Films via Metal-Ion Intercalation.

VO2 has shown great promise for sensors, smart windows, and energy storage devices, because of its drastic semiconductor-to-metal transition (SMT) near 340 K coupled with a structural transition. To push its application toward room-temperature, effective transition temperature (Tc) tuning in VO2 is desired. In this study, tailorable SMT characteristics in VO2 films have been achieved by the electrochemical intercalation of foreign ions (e.g., Li ions). By controlling the relative potential with respect to Li/Li+ during the intercalation process, Tc of VO2 can be effectively and systematically tuned in the window from 326.7 to 340.8 K. The effective Tc tuning could be attributed to the observed strain and lattice distortion and the change of the charge carrier density in VO2 introduced by the intercalation process. This demonstration opens up a new approach in tuning the VO2 phase transition toward room-temperature device applications and enables future real-time phase change property tuning.

Capacitive NO2 Detection Using CVD Graphene-Based Device.

A graphene-based capacitive NO2 sensing device was developed by utilizing the quantum capacitance effect. We have used a graphene field-effect transistor (G-FET) device whose geometrical capacitance is enhanced by incorporating an aluminum back-gate electrode with a naturally oxidized aluminum surface as an insulating layer. When the graphene, the top-side of the device, is exposed to NO2, the quantum capacitance of graphene and, thus, the measured capacitance of the device, changed in accordance with NO2 concentrations ranging from 1-100 parts per million (ppm). The operational principle of the proposed system is also explained with the changes in gate voltage-dependent capacitance of the G-FET exposed to various concentrations of NO2. Further analyses regarding carrier density changes and potential variances under various concentrations of NO2 are also presented to strengthen the argument. The results demonstrate the feasibility of capacitive NO2 sensing using graphene and the operational principle of capacitive NO2 sensing.

Observation of resonant exciton and correlated plasmon yielding correlated plexciton in amorphous silicon with various hydrogen content

Hydrogenated amorphous silicon (a-Si: H) has received great attention for rich fundamental physics and potentially inexpensive solar cells. Here, we observe new resonant excitons and correlated plasmons tunable via hydrogen content in a-Si: H films on Indium Tin Oxide (ITO) substrate. Spectroscopic ellipsometry supported with High Resolution-Transmission Electron Microscopy (HR-TEM) is used to probe optical properties and the density of electronic states in the various crystallinity from nano-size crystals to amorphous a-Si: H films. The observed optical and electronic structures are analyzed by the second derivative with analytic critical-point line shapes. The complex dielectric function shows good agreement with microscopic calculations for the energy shift and the broadening inter-band transitions based on the electron–hole interaction. Interestingly, we observe an unusual spectral weight transfer over a broad energy range revealing electronic correlations that cause a drastic change in the charge carrier density and determine the photovoltaic performance. Furthermore, the interplay of resonant excitons and correlated plasmons is discussed in term of a correlated plexciton. Our result shows the important role of hydrogen in determining the coupling of excitons and plasmons in a-Si: H film for photovoltaic devices.

Size effect of gold nanoparticles in bimetallic ZIF catalysts for enhanced photo-redox reactions

It is well known that gold nanoparticles have been widely used in various photo-redox reactions because of the unique surface plasmon resonance effect and the excellent internal electron transfer ability. However, the essential factors of how the metal nanoparticles enhancing the activity of photocatalysts have not been fully revealed. In this paper, gold colloidal particles of different sizes were modified on the surface of the dodecahedral skeleton of ZIF-Co/Zn. The photocatalytic activity of Au@ZIF-Co/Zn was investigated under simulated visible light for treating Rhodamine B/Cr(VI) mixture solutions (RhB/Cr(VI) 20 mg/L) and 4-nitrophenol solution (4-NP 20 mg/L). The results indicate that the size of gold nanoparticles could affect the surface plasmon resonance effect observably, internal electron transfer capacity and carrier density change. Photoelectric chemistry experiments and radical quenching experiments were performed to reveal the charge-transfer photocatalytic mechanism and size influence of gold nanoparticles in composite photocatalysts. This work may provide a novel strategy for the construction of metal–organic frameworks for the purification of wastewater mixed with heavy metal ions, organic pollutants and pharmaceutical intermediates.

Active metasurface in the near-infrared region by gating ultrathin TiN films.

Ultrathin titanium nitride (TiN) films have become a novel material flatform for constructing active metasurfaces in the near-infrared region. In this Letter, we numerically achieved the dual functions of switchable linear dichroism (LD) and tunable perfect absorption in a G-shape gold resonators/TiN film hybrid metasurface by gating ultrathin TiN films. As the carrier density of TiN decreases, the modulation depth for LD strength is about 70% at 1211 nm. Meanwhile, the response wavelength of perfect absorption (∼1) shifts to the blue by around 130 nm with a change of carrier density of 12%. Our proposed active metasurface with the capability of strength-switchable LD and wavelength-tunable perfect absorption has considerable potential in dynamic electro-optic modulation and flat photonic devices with reconfigurable functionalities.

Gate-tunable modulation of the optical properties of multilayer graphene by the reversible intercalation of ionic liquid anions

We demonstrate a substantial modulation of the optical properties of multilayer graphene (∼100 layers) using a simple device consisting of a multilayer graphene/polymer electrolyte membrane/gold film stack. Applying a voltage of 3–4 V drives the intercalation of anion [TFSI]− [ion liquid diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide [DEME][TFSI]] resulting in the reversible modulation of the properties of this optically dense material. Upon intercalation, we observe an abrupt shift of 35 cm−1 in the G band Raman mode, an abrupt increase in FTIR reflectance over the wavelength range from 1.67 to 5 μm (2000–6000 cm−1), and an abrupt increase in luminescent background observed in the Raman spectra of graphene. All of these abrupt changes in the optical properties of this material arise from the intercalation of the TFSI− ion and the associated change in the free carrier density (Δn = 1020 cm−3). Suppression of the 2D band Raman mode observed around 3 V corresponds to Pauli blocking of the double resonance Raman process and indicates a modulation of the Fermi energy of ΔEF = 1.1 eV.

Strain Driven Conducting Domain Walls in a Mott Insulator

AbstractRewritable nanoelectronics offer new perspectives and potential to both fundamental research and technological applications. Such interest has driven the research focus into conducting domain walls: pseudo‐2D conducting channels that can be created, positioned, and deleted in situ. However, the study of conductive domain walls is largely limited to wide‐gap ferroelectrics, where the conductivity typically arises from changes in charge carrier density, due to screening charge accumulation at polar discontinuities. This work shows that, in narrow‐gap correlated insulators with strong charge‐lattice coupling, local strain gradients can drive enhanced conductivity at the domain walls—removing polar‐discontinuities as a criteria for conductivity. By combining different scanning probe microscopy techniques, it is demonstrated that the domain wall conductivity in GaV4S8 does not follow the established screening charge model but likely arises from the large surface reconstruction across the Jahn–Teller transition and the associated strain gradients across the domain walls. This mechanism could turn any structural, or even magnetic, domain wall conducting, if the electronic structure of the host is susceptible to local strain gradients—drastically expanding the range of materials and phenomena that may be applicable to domain wall‐based nanoelectronics.

Large Optical Tunability of 5d 2D Electron Gas at the Spinel/Perovskite γ‐Al2O3/KTaO3 Heterointerface

AbstractThe 5d 2D electron gas (2DEG) in KTaO3‐based heterostructures exhibits stronger spin‐orbit coupling and higher superconducting transition temperature compared to the 3d SrTiO3‐based 2DEG, thus attracts much attention recently. However, compared to the intensively investigated isostructural perovskite‐type interfaces, the non‐isostructural 5d oxide interfaces remain less investigated. Herein, for the first‐time, epitaxial spinel/perovskite γ‐Al2O3/KTaO3 heterointerface is created, at a deposition temperature as low as 300 °C. Metallic 2DEG emerges at the interface when the thickness of γ‐Al2O3 overlayer exceeds a critical thickness of approximately 2.4 nm. The interface states are found to be tuned largely by the light illumination, and the maximum change in the 2DEG carrier density under the light is 3 × 1013 cm–2. The light‐tunable spin‐orbit coupling exhibits a maximum strength of Rashba spin‐orbit coupling and spin‐splitting energy of ≈ 7.93 × 10–12 eV m and ≈ 24.96 meV, respectively. Such remarkable photosensitivity of the non‐isostructural 5d 2DEG offers new opportunities for oxide optoelectronic devices.

Solvable theory of a strange metal at the breakdown of a heavy Fermi liquid

We introduce an effective theory for quantum critical points (QCPs) in heavy fermion systems, involving a change in carrier density without symmetry breaking. Our new theory captures a strongly coupled metallic QCP, leading to robust marginal Fermi liquid transport phenomenology, and associated linear in temperature ($T$) "strange metal" resistivity, all within a controlled large $N$ limit. In the parameter regime of strong damping of emergent bosonic excitations, the QCP also displays a near-universal "Planckian" transport lifetime, $\tau_{\mathrm{tr}}\sim\hbar/(k_BT)$. This is contrasted with the conventional so-called "slave boson" theory of the Kondo breakdown, where the large $N$ limit describes a weak coupling fixed point and non-trivial transport behavior may only be obtained through uncontrolled $1/N$ corrections. We also compute the weak-field Hall coefficient within the effective model as the system is tuned across the transition. We further find that between the two plateaus, reflecting the different carrier densities in the two Fermi liquid phases, the Hall coefficient can develop a peak in the critical crossover regime, like in recent experimental findings, in the parameter regime of weak boson damping.