Bolometric Applications Research Articles

Fluorinated Graphene Films with High Temperature Sensitivity for Uncooled Bolometer Applications

Fluorinated Graphene Films with High Temperature Sensitivity for Uncooled Bolometer Applications

Theoretical simulation of pre-sintering effect and improvement of electrical transport properties of La0.7Ca0.18Sr0.12MnO3 film prepared by spin coating method

Perovskite manganites Lax(Ca1-ySry)1-xMnO3 have drawn the plentiful attention owing to their distinctive physical properties. Their unique properties make them an ideal candidate for high-sensitivity and uncooled bolometric applications. Optimized La0.7Ca0.18Sr0.12MnO3 (LCSMO) films were grown on (00l) LaAlO3 substrate at different pre-sintering temperature (Tps) via sol–gel spin coating method. According to the test results of microstructure, internal strain, Mn ion valences and surface morphology, electrical transport properties are restricted by MnO6 octahedron distortion and film crystallinity affected by the change in Tps. The microstructure growth was improved and the surface morphology tended to grow in a cross-like structure with high crystallinity by optimizing Tps. Double exchange mechanism and internal strain were enhanced with the increase in Tps. Thereby, due to the enlarging MnOMn bond angle and enhancing MnO covalency, the temperature (Tk) corresponding to temperature coefficient of resistivity (TCR) was increased gradually as Tps increased. The TCR value of LCSMO film reached a maximum value (10.82 %/K) and Tk reached room temperature (302.03 K) at Tps = 1123 K. The theoretical foundation of the pre-sintering effect has been supplemented, helping to understand the influence of pre-sintering on film growth and electrical transport properties.

Bias-tunable temperature coefficient of resistance in Ge transistors

Ge-based bolometers are widely used for near-infrared detection for a broad range of applications such as thermography or chemical analysis. Notably, for the thermometers used in bolometers, integration, scaling, and sensitivity as well as functionality are of utmost importance. In this regard, Ge exhibits a favorable temperature sensitivity due to the relatively low bandgap and a high intrinsic charge carrier concentration. In this work, we demonstrate a nanoscale thermometer for bolometric applications on the base of Ge-on-insulator nanosheets with monolithic Al source/drain contacts envisioned for future wafer-scale integration. Importantly, electrostatic gating of the nanosheets allows the operation as a Schottky barrier field-effect transistor, providing tunability of the energy landscape and the involved charge carrier injection in interaction with the metal-semiconductor junctions. In this approach, the top-gate electrode and drain contact are connected, thus resembling a two-terminal device with bias-tunable temperature coefficient of resistance (TCR) values between 0%/K and −3.8%/K in the temperature range of T = 125–150 K. Moreover, in this configuration, even at room temperature, a maximum TCR value of −1.6%/K is achieved. The bias-tunable TCR exhibited in these devices may enable advanced concepts for room temperature bolometric applications and allow co-integration with nanoelectronics.

Large temperature coefficient of resistivity (TCR) of La0.67Ca0.33MnO3 polycrystalline ceramics improved by optimizing sintering temperatures

This work investigates fabrication of La0.67Ca0.33MnO3 (LCMO) polycrystalline ceramics using a facile sol–gel method at sintering temperatures (Ts) ranging from 1470 °C to 1540 °C and explores relationship between their electrical transport properties and Ts. X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy, and X-ray photoemission spectroscopy were employed to characterize crystal structure, morphologies, element distributions, and Mn3+/Mn4+ ratio of the as-obtained LCMO ceramics. The influence of Ts on phase purity, microstructure, morphology, and electrical transport properties was also investigated. LCMO ceramics exhibit increased grain size and higher temperature coefficient of resistivity (TCR) at higher temperatures. An increase in Ts can reduce the strength of electron–lattice interactions and theoretical Curie temperature in LCMO ceramic samples, which can in turn optimize their electrical transport properties. Moreover, an increase in Ts enhances double-exchange (DE) effect to some extent. The DE effect further affects resistivity and metal–insulator transition temperature of material. Additionally, small-polaron hopping model and phenomenological percolation model were used to characterize electrical transport in metal–insulator transition region. The temperature at which TCR is maximum (265.78 K) and the largest TCR value (58.16 %) were achieved at a Ts of 1530 °C. By modulating the sintering temperature, we achieved LCMO ceramics with larger grains and enhanced electrical properties. This optimization renders materials suitable for highly sensitive infrared bolometer applications.

Investigation of optical, dielectric properties and conduction mechanism of LiCo0.7Mn0.3O2

The development of a multifunctional material with variety in its properties is a powerful research project. The electrochemical properties of LiCo0.7Mn0.3O2 material have attracted our attention to search electrical characterization and the optical properties of this material. The synthesis of this compound is based on a solid state method. The x-ray powder diffraction analysis shows that the compound crystallizes in the hexagonal system with the R −3 m space group. Moreover, the homogenous distribution of grain is revealed by the EDX study, thus the grain size is about 2.5 μm calculated from the scanning electron microscopy data. The band gap energy was established and seems to be equal to 1.88 eV which confirms the semiconductor character of this compound. Impedance spectroscopy was performed in the temperature ranging from 363 K to 473 K and frequency varying between 0.1 to 106Hz. The Nyquist plots confirm the presence of grains and grain boundary contribution instead of electrode polarization. The obtained conductivity properties indicates the semiconductor behavior of our compound, also it confirms its reliability for electrochemical application. Ac conductivity has been adjusted using the Jonsher power law, which allows us to confirm that the dc conductivity is thermally activated with activation energy of 400 meV and 500 meV for 363–423 K, 423–473 K regions, respectively. Temperature dependence of the exponent s reveals that the conduction process is governed by the correlated barrier hopping model (CBH). Besides, the temperature coefficient of resistivity (TCR) affirmed that LiCo0.7Mn0.3O2 is a good candidate for bolometric applications.

Symmetric Time-Division-Multiplexed SQUID Readout With Two-Layer Switches for Future TES Observatories

Time-division multiplexing (TDM) of transition-edge-sensor (TES) microcalorimeters is being developed as the readout technology for the Athena X-ray integral field unit (X-IFU) and CMB-S4. We present an experimental demonstration of our latest TDM architecture, which has been implemented in a 4-column × 34-row chip that is fully compatible with the X-IFU design specifications. This new “mux21” architecture is designed for differential readout and uses two-layer switches to reduce the number of row-address lines while also incorporating changes that reduce power to roughly 40% and the total number of SQUIDs to half that of TDM chips used in previous publications. With the 160 ns row durations specified for X-IFU, we obtained (2.02 ± 0.03) eV FWHM energy resolution at 5.9 keV in 2-column × 34-row TDM readout of a NASA X-IFU-like TES array, which meets the requirements of X-IFU's energy-resolution budget. Finally, a scaling analysis of our improved TDM readout chain to bolometer applications indicates that multiplexing factors on the scale of several hundred are possible, depending on the readout requirements.

Low temperature synthesis of VO2 and hysteresis free VOx thin films with high temperature coefficient of resistance for bolometer applications

Vanadium dioxide (VO2) is an insulator to metal phase transition material, which finds many electronic and optical applications due to its reversible switching while VOx-based bolometers are widely used in commercial uncooled infrared detectors due to their high-temperature coefficient of resistance and low resistivity. Atmospheric pressure thermal oxidation (APTO) of Vanadium (V) is a simple process used to synthesize VO2 films without the need for precise control of oxygen partial pressure unlike other synthesis methods. In this work, we report the effect of reducing the V oxidation temperature to 250–450 °C during APTO and synthesize films with different oxidation durations. For an optimized oxidation duration at each oxidation temperature, VO2 films are obtained with more than two orders of resistance switching. This optimum oxidation duration is found to decrease exponentially with increase in oxidation temperature with an activation energy of 1.05 eV. The phase transition temperature of these VO2 films is found to decrease monotonically with decrease in oxidation temperature. We also observe that partially oxidized (oxidation time less than optimized oxidation duration to form VO2) VOx films show broad resistance switching from room temperature up to 70 °C instead of steep switching at the phase transition temperature. The partially oxidized film shows a temperature coefficient of resistance (TCR) as high as ∼10%/K, which is 3–4 times greater than TCR of commercial VOx thin films used in bolometers while showing similar resistivity. The resistivity of these films also exhibits almost zero hysteresis with temperature, an important requirement for bolometer applications. This study will widen the usefulness of the APTO process for fabricating VO2-based devices and high TCR VOx films for bolometer applications. The demonstrated low temperature process will especially be suitable to integrate VO2/VOx on substrates which can sustain only low temperatures such as some of the flexible substrates.

Shockproof Deformable Infrared Radiation Sensors Based on a Polymeric Rubber and Organic Semiconductor H2Pc-CNT Composite.

Polymeric rubber and organic semiconductor H2Pc-CNT-composite-based surface- and sandwich-type shockproof deformable infrared radiation (IR) sensors were fabricated using a rubbing-in technique. CNT and CNT-H2Pc (30:70 wt.%) composite layers were deposited on a polymeric rubber substrate as electrodes and active layers, respectively. Under the effect of IR irradiation (0 to 3700 W/m2), the resistance and the impedance of the surface-type sensors decreased up to 1.49 and 1.36 times, respectively. In the same conditions, the resistance and the impedance of the sandwich-type sensors decreased up to 1.46 and 1.35 times, respectively. The temperature coefficients of resistance (TCR) of the surface- and sandwich-type sensors are 1.2 and 1.1, respectively. The novel ratio of the H2Pc-CNT composite ingredients and comparably high value of the TCR make the devices attractive for bolometric applications meant to measure the intensity of infrared radiation. Moreover, given their easy fabrication and low-cost materials, the fabricated devices have great potential for commercialization.

Magneto transport behavior of La0.67Pb0.33MnO3manganite system with various additives

The nanostructured manganites show low field magnetoresistance, where grain boundary effects influence the phenomenon. The current investigation aims to comprehend the impact of the grain boundaries on the magnetic and magneto transport characteristics of La0.67Pb0.33MnO3 manganite with 1% of additives ZrO2, Bi2O3, and Li2CO3. Two step synthesis processes was adopted to prepare the pure and additive samples. The samples were characterized with XRD, XPS and FESEM to establish the structure, phase purity, oxidation states, elemental analysis and morphology respectively. The prepared samples were formed in single-phase rhombohedral structure with the enhanced crystallite and grain size with additives when compared to the pure sample. The observed magnetic transition temperature, are well above room temperature (TC = 353 K) and a soft ferromagnetic nature with 70% of ferromagnetic fraction is noticed at T = 300 K for all the studied samples. The metal insulator transition temperature (TP) remains invariant for ZrO2, Bi2O3 additives, while it reduces for Li2CO3 additive. It is observed that 1% of additives are acting as conduction barriers at the grain boundaries as a consequence there is an increase in resistivity. The contribution to magnetoresistance is seen as intrinsic and extrinsic in pure and additive materials exhibiting high percentage both at low and room temperature. All the sample shows an appreciable temperature coefficient of resistance percentage at their respective transition temperatures which can be made useful for bolometric applications. Conduction mechanisms in the ferromagnetic and paramagnetic phases were examined with the electron-magnon scattering and small polaron hopping model respectively.

Epitaxial Grown VO2 with Suppressed Hysteresis and Low Room Temperature Resistivity for High-Performance Thermal Sensor Applications

The ability of VO2 to undergo semiconductor-to-metal phase transition (SMT) upon heating makes it a very attractive material for uncooled bolometers. The SMT of VO2 represents a large temperature coefficient of resistance, which is an important parameter for the development of highly responsive microbolometers. However, other characteristics of the SMT of VO2 such as its high transition temperature (341.2 K), the sharpness of the transition, its hysteresis, and the high room temperature resistivity limit the performance of this material in microbolometers. In this work, we grow a high-quality epitaxial ultrathin film VO2 on c-plane Al2O3 by pulsed laser deposition. The low deposition temperature and tuning the oxygen partial pressure during the growth process enable control over the grain size and oxygen vacancy concentration. This allowed controlling the SMT parameters of the samples. In particular, we show that the high density of grain boundaries associated with nanosized grains suppresses the thermal hysteresis of the SMT. Simultaneous control over the density of oxygen vacancies and the size of grains enables the adjustment of the temperature coefficient of resistance, room temperature resistivity, SMT temperature, sharpness, and thermal hysteresis toward suitable values for the fabrication of efficient VO2-based uncooled bolometers. Compared with other VO2 fabrication methods, this approach can be viewed as a simpler alternative for VO2 fabrication with favorable properties for practical bolometer applications.

Investigation of Structural and Infrared Characteristics of Silicon Nanowires for Bolometric Application

Investigation of Structural and Infrared Characteristics of Silicon Nanowires for Bolometric Application

Metal-carbon nanotube composite for wavelength-selective bolometer with improved characteristics

This work comprehensively discusses the utilization of a metal-multiwalled carbon nanotube (CNT) nanocomposite of gold (Au)/CNT for uncooled bolometric application synthesized by a simple rapid green synthesis technique. Enhanced light–matter interaction with the visible (532 nm) and near-infrared (1064 nm) radiations in the nanocomposite is utilized for achieving improved bolometric response at room temperature (RT, ∼300 K). A comparatively higher temperature coefficient of resistance of ∼−0.11%/K (improvement of ∼57%) was achieved in the Au/CNT nanocomposite bolometer (BM). Additionally, the composite BM demonstrated a dual (532 and 1064 nm) spectral selectively and enhanced sensitivity with respective large RT voltage responsivities of ∼11.70 ± 3.45 V/W and ∼503.54 ± 12.77 mV/W, as opposed to null response from the CNT BM. The current study, thus, demonstrates an important step toward designing high-performing CNT BMs for uncooled operation, with added functionality of wavelength-selectiveness.

Summerfield scaling model and electrical conductivity study for understanding transport mechanisms of a Cr3+ substituted ZnAl2O4 ceramic.

Solid-state and sol-gel procedures were used to prepare ZnAl1.95Cr0.05O4 nanocrystal spinels. From the results obtained by X-ray diffraction (XRD) and transmission electron microscopy (TEM), it can be concluded that the samples prepared by sol-gel synthesis are better crystallized than the ones resulting from the solid-state method. Studies by spectroscopy of impedance were done in function of frequency (40-107 Hz) and temperature (540-680 K) in the sample prepared by sol-gel synthesis. The electrical conductivity spectra obey Jonscher's law and two models were observed studying the variation of the exponent 's' as a function of temperature, Correlated Barrier Hopping (CBH) and Non-overlapping Small Polaron Tunnelling (NSPT). The predominant conduction mechanism is bipolaron hopping. The scaling behavior of conductivity spectra was checked by Summerfield scaling laws. The time-temperature superposition principle (TTSP) points to a common transport mechanism working for the low and middle frequency ranges. The scaling mechanism fails in the high-frequency ranges suggesting that conduction dynamics, and the usual hopping distance of mobile species, have changed. The values obtained for the activation energy from the hopping frequency, conductivity σ dc, bulk resistance R gb, and relaxation (f max), in the temperature range of 540-680 K, are very close. A higher and negative temperature coefficient of resistivity (TCR coefficient) equal to -2.7% K-1 is found at 560 K. This result shows that our compound is suitable for uncooled infrared bolometric applications and infrared detectors.

Compositional, electrical and thermal properties of nonstoichiometric titanium oxide thin films for MEMS bolometer applications

Nonstoichiometric titanium-oxide (TiOx) films are being extensively studied for MEMS bolometer applications. To have superior bolometer performance, the TiOx layers should have a high value of temperature coefficient of resistivity (TCR) and a low value of thermal conductivity. This paper presents the evolution of the electrical and thermal properties of RF sputtered TiOx films with the film thickness and stoichiometry. A series of amorphous TiOx films of 100 nm, 200 nm, and 400 nm thickness are deposited with different oxygen flow rates. X-ray photoelectron spectroscopy (XPS) study revealed the presence of only Ti4+ and Ti3+ oxidation states in the deposited films, and no TiO phase exists (no Ti2+ state). The oxygen-titanium atomic ratios (O/Ti) of the films are 1.5, 1.7, and 1.9. The TCR of the films is increased (from -1.1 %/K to -4.8%/K) with the increased oxygen content. The cross-plane thermal conductivities of the films are by the 3-ω technique. The thermal conductivity of the TiOx films is increased (0.5 W/m-K to 2.4 W/m-K) with the increase in film thickness and is independent of its ‘x’ values. Characteristics of MEMS bolometer pixel (17 mm pitch) are simulated based on the measured properties of the TiOx films. The film having 2.75 %/K TCR and 1.12 W/m-K thermal conductivity, exhibited minimum NETD (37.3 mK) with decent voltage responsivity (1.62x106 V/W).

La0.7Ca0.3MnO3-δ:Ag nanocomposite thin films with large temperature coefficient of resistance (TCR)

In this work, oxygen vacancy-regulated La0.7Ca0.3MnO3-δ:Ag (LCMO:A) nanocomposite thin films on LaAlO3 (001) substrates were investigated to obtain films with large temperature coefficient of resistance (TCR) values. LCMO:A nanocomposite thin films were synthesized using pulsed laser deposition, and oxygen pressures during film deposition and annealing steps were optimized. As oxygen pressures increased, lattice parameter increased from 70 Pa to 100 Pa, Tp increased monotonically from 168 K to 282 K, and average Mn4+ concentration in the film increased as indicated by X-ray photoemission spectroscopy data. Record high TCR value of ∼37% K−1 was achieved in LCMO:A nanocomposite thin film prepared with optimal oxygen pressures, making this film promising candidate for applications in bolometers.

Investigation of structural, morphological, and transport properties of a multifunctional Li-ferrite compound.

The development of multifunctional materials is an exceptional research area, which is aimed at enhancing the versatility of materials in various applications. In this context, the exceptional properties of ferrite materials have attracted the attention of researchers. For this reason, we synthesized LiMn0.5Fe2O4 sintered at a temperature of 1100 °C. The X-ray powder diffraction analysis reveals the presence of one cubic phase with the Fd3̄m space group and confirms the spinel structure formation. Moreover, the elemental analysis by EDX reveals the homogeneous distribution of iron and manganese cations. Scanning electron microscopy shows that the grain size is of the order of 2.48 μm. Impedance spectroscopy was performed in the temperature and frequency ranges from 200 K to 380 K and 40 Hz to 106 Hz, respectively. The Nyquist plots revealed the presence of grains and grain boundary contributions. The semiconductor nature, obtained by the conductivity study, indicates that LiMn0.5Fe2O4 is promising in optoelectronic applications. Dc conductivity is found to be thermally activated with an activation energy of 370 meV, 255 meV, and 199 meV for 200–270 K, 280–330 K, and 340–380 K regions, respectively. From the Jonscher power law, the correlated barrier hopping model (CBH) and non-overlapping small polaron tunneling (NSPT) prevailed in the conduction process. Besides, the temperature coefficient of resistivity (TCR) affirmed that LiMn0.5Fe2O4 is a good candidate for detecting infrared radiations and infrared bolometric applications.

Improved room-temperature TCR of La0.7Ag0.125K0.175MnO3 films by optimizing sintering temperatures

Cation-doped perovskite manganite has unique electrical transport properties; therefore, it has attracted attention due to its promising applications in the field of uncooled infrared bolometers. For the bolometer applications, high temperature coefficient of resistivity (TCR) at room-temperature is essential for the perovskite materials. To investigate the effect of sintering temperature (Ts) on TCR and obtain high TCR values of manganite films at room temperature, La0.7Ag0.125K0.175MnO3 (LAKMO) films were prepared on the (001) SrTiO3 substrate at various Ts via sol–gel spin coating method. Using analysis of structure, morphology, ionic valence, and electrical transport properties, LAKMO films with better crystalline quality and larger TCR values were obtained at higher Ts. The TCR value of LAKMO films reached a maximum value (9.46 % K−1) and the TCR peak temperature (Tk) reached room-temperature (298.75 K) at Ts of 1000 °C, indicating that the material has great potential for the uncooled infrared bolometer applications.

Growth evolution and infrared response of thermally dewetted Au nano-structures for bolometric applications

This paper discusses the growth and characterization of thermally dewetted gold nanostructures for enhanced infrared absorption in bolometer applications. The Au nanostructures are fabricated by solid-state dewetting of ultrathin (18 nm) Au film on Si substrate. The duration of dewetting is varied from 1 min to 30 min. The Au film exhibited the highest X-ray diffraction (XRD) peak intensity along the (111) plane; while the Au nanostructure showed the rising XRD peak along the (220) plane with the dewetting duration. The dewetting process also yielded grain growth and recrystallization of Au nanostructure. The effect of the dewetting on surface morphologies of the Au nanostructure is studied. The Au nanostructures exhibited broadband infrared absorption in the 500–6500 cm−1 range. The sample dewetted for 10 min exhibited up to 200% enhanced absorbance (with respect to the silicon substrate) in the IR range due to the well-suited dimensions of the nanostructures. The hypothesis is also substantiated by examining the plasmonic response of the nanostructure by simulation.

A microwave SQUID multiplexer optimized for bolometric applications

A microwave superconducting quantum interference device multiplexer has been optimized for reading out large arrays of superconducting transition-edge sensor (TES) bolometers. We present the scalable cryogenic multiplexer chip design that may be used to construct an 1820-channel multiplexer for the 4–8 GHz rf band. The key metrics of yield, sensitivity, and crosstalk are determined through measurements of 455 readout channels, which span 4–5 GHz. The median white-noise level is 45 pA/Hz, evaluated at 2 Hz, with a 1/f knee ≤ 20 mHz after common-mode subtraction. The white-noise level decreases the sensitivity of a TES bolometer optimized for detection of the cosmic microwave background at 150 GHz by only 3%. The measured crosstalk between any channel pair is ≤ 0.3%.

Fabrication and characterization of Al-Mn superconducting films for applications in TES bolometers* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11653001 and 11653004).

Superconducting transition edge sensor (TES) bolometers require superconducting films to have controllable transition temperatures T c in different practical applications. The value of T c strongly affects thermal conductivity and thermal noise performance of TES detectors. Al films doped with Mn (Al–Mn) of different concentrations can accomplish tunable T c. A magnetron sputtering machine is used to deposit the Al–Mn films in this study. Fabrication parameters including sputtering pressure and annealing process are studied and their influences on T c and superconducting transition width ΔT c are optimized. The Al–Mn films with ΔT c below 1.0 mK for T c in a range of 520 mK–580 mK are successfully fabricated.