Although black phosphorous (BP) is a promising two-dimensional material for next-generation infrared (IR) photodetectors, enhancing its quantum efficiency remains challenging. Herein, we proposed a hybrid BP–plasmonic nanograting with high aspect ratios for use in advanced functional IR photodetectors. Plasmonic nanogratings with high aspect ratios exhibit wide-angle near-unity absorption in the IR-wavelength region; this absorption is based on the grating depth. By forming BP on top of the plasmonic nanogratings and modifying the edge structure of BP, we achieved a hybrid structure that exhibits strong absorption. The results demonstrate the viability of BP-based high-performance IR photodetectors.
Lenses for use in the infrared (IR) wavelength region are key elements of IR image sensors, and determine the cost and performance of such sensors. As an alternative to a conventional lens, we designed and fabricated a silicon-based metalens in order to realize a compact, high-performance IR focal plane array (IRFPA) system. The metalens was developed using an automatic inverse design technique based on a rigorous coupled wavelength analysis. It consisted of a periodic array of pillars fabricated by deep reactive-ion etching of a silicon wafer. This metalens was designed for the 80×60 IRFPA used in the Mitsubishi Electric Corporation MIR8060B1, which comprises an array of pn-junction diodes formed on a silicon-on-insulator layer. It is expected that this metalens-based IR sensor will expand the range of applications of such sensors.
Graphene/semiconductor heterojunction photodiodes that use photogating are expected to perform better than conventional infrared (IR) photodetectors. However, interface instability limits has prevented the realization of the theoretically predicted performance and high reliability for these devices. This study focuses on optimizing the material and thickness of an interfacial layer in a graphene/InSb heterojunction to achieve a high-performance mid-IR photodetector. The results indicate that HfO2 is a more suitable material than Al2O3 for the interfacial layer, and 1-2 nm is the thickness that best promotes effective photocurrent transport. This interfacial layer can facilitate the fabrication of superior IR image sensors based on graphene/InSb heterojunctions.
Graphene-based infrared photodetectors are promising devices that exploit the unique optoelectronic properties of graphene, including its broadband light absorption characteristics, rapid response, and high chemical stability. However, graphene exhibits a low absorbance (2.3%), which limits its photoresponsivity. This paper introduces sensitivity-enhanced InAs/GaInSb type-II superlattice (T2SL) infrared photodetectors fabricated using a graphene diode structure. The devices consist of graphene diodes and InAs/GaInSb SLs grown via chemical vapor deposition. The T2SL structure is employed for both photocarrier supply source and carrier density modulation of the graphene diodes to improve the sensitivity of the devices. The dark current in the graphene diode device is reduced to less than 1%, which is lower than that in the GFET device, and the responsivity of the devices is significantly enhanced using the photogating effect. These highly sensitive and low-dark-current devices are expected to promote the development of high-performance graphene-based image sensors.
Graphene/InSb heterojunction mid-infrared photodiodes for infrared image sensors exhibit high responsivity, low noise, and excellent detection performance. However, the performance of each pixel varies owing to the instability of the graphene/InSb interface state. In this study, the performance variation was addressed by inserting an interfacial layer at the graphene/InSb interface. A few-nanometer-thick HfO2 interfacial layer was inserted at the graphene/InSb interface. Compared to devices without an interfacial layer, those with an interfacial layer exhibited greatly improved minimum noise equivalent temperature difference (NETD) and pixel-by-pixel NETD variation. This was due to the stabilization of the InSb surface and increased photocurrent caused by photoswitching, which significantly changed the Fermi level of the graphene. The insertion of the HfO2 interfacial layer improved the standard deviation to less than 1/30. Thus, inserting an interfacial layer at the graphene/InSb interface is expected to facilitate the development of high-performance infrared image sensors with low variability.
Graphene infrared (IR) photodetectors are promising devices that exploit the unique optoelectronic properties of graphene, such as broadband light absorption, rapid response, and high-chemical stability. However, graphene has low absorbance (2.3%), which limits its photo-responsivity. This study investigated the middle-wavelength infrared (MWIR) and long-wavelength IR (LWIR) responsivity enhancements in graphene photodetectors with type-II superlattices (T2SL) employed as photosensitizers. Graphene field-effect transistors (GFETs) were fabricated with the InAs/InGaSb SL photosensitizers on a GaSb substrate. The device was vacuum-cooled and then exposed to light using a filament lamp (wavelength ranges: 3–5 μm and 8–12 μm). The device exhibited MWIR and LWIR photoresponses at 77 K, whereas no photoresponse was observed when photosensitizers were not used. The observed current variations in the photosensitizers and the substrate suggest that photocarriers in the InAs/InGaSb SL modulate the gate voltages on the GFETs, thereby producing the photogating effect. The LWIR photoresponse in conjunction with the photogating effect was enhanced by a factor of 1500 compared with that without the photosensitizers. The results obtained in this study are expected to contribute to the development of high-performance graphene-based IR image sensors.
Black phosphorous (BP) is a promising material for infrared (IR) photodetectors. We theoretically investigated and proposed BP/graphene van der Waals (vdW) heterostructure-based metasurface wavelength-selective IR photodetectors that operate on the basis of plasmonic resonance. The proposed structure consists of a top layer consisting of periodic monolayers of BP, a graphene-based vdW heterostructure layer on a middle dielectric layer, and a bottom reflector. The periodic BP produces polarization-dependent localized surface plasmon resonance (LSPR) according to its armchair or zigzag edge, resulting in strong wavelength-selective absorption. The graphene layer can enhance the LSPR of BP, inducing propagating surface plasmon resonance on graphene. These results will contribute to the realization high-performance BP/graphene-based IR photodetectors.
Graphene nanoribbons (GNRs) and graphene hybrid photodetectors were demonstrated in the middle- and long-wavelength infrared (MWIR and LWIR, respectively) regions. Graphene transistors were prepared using Si substrates with an SiO2 layer and source and drain electrodes. Single-layer graphene fabricated by chemical vapor deposition was transferred onto the substrates to form a channel; the GNR was formed on this channel by solution dispersion. The formation of graphene and the GNR was confirmed by position mapping of the Raman spectra. The photoresponse was measured in the MWIR and LWIR regions, and was found to be drastically enhanced for devices with the GNR when compared with those without it. Although the devices without the GNR could not respond at temperatures higher than 15 K, those with the GNR could be operated at temperatures up to 150 K. This was attributed to photogating by the GNR layers that absorbed the MWIR and LWIR radiation, leading to a significant temperature change. These results can potentially contribute toward developing high-performance and broadband IR graphene-based photodetectors.
In this study, room-temperature long-wavelength infrared (LWIR) graphene photodetectors using pyroelectric photosensitizers are demonstrated. The devices comprise graphene-based field-effect transistors with lithium niobate as a pyroelectric photosensitizer. To enhance the photodetector performance, we studied the influence of photosensitizer thickness and ferroelectric polarization direction on the pyroelectric photogating effect. Modulation of the gate current indicates that the IR photoresponse is considerably amplified by thick photosensitizers in which dielectric polarization occurs in the direction perpendicular to the graphene channel. The results will contribute to the development of high-performance graphene LWIR image sensors.
Herein, we developed a high-performance graphene/InSb heterojunction mid-infrared (IR) photogated diode for IR image sensors. We achieved low noise owing to a significant reduction in the dark current and high responsivity, which is attributed to the graphene/InSb heterojunction diode structure and the photogating effect. The detection performance of the proposed device is better than that of conventional graphene-based IR sensors in the mid-infrared region of 3–5 μm. These results indicate that the combination of a simple graphene/InSb heterojunction and the photogating effect can produce IR image sensors with better detection performance than existing IR sensors.
We theoretically propose van der Waals (vdW) heterostructure-based wavelength selective infrared (IR) photodetectors using plasmonic metasurfaces (PMs), consisting of top micropatches, graphene on a hexagonal boron nitride (hBN) vdW heterostructure layer, and a bottom reflector. The hBN ensures the vdW heterostructures maintain high carrier mobility in the graphene. The wavelength selective detection of the graphene can be controlled mainly by the micropatch size over a wide range from the middle- to long-wavelength IR regions. The wavelength can also be electrically tuned by the chemical potential of the graphene. These results will contribute to developing high-performance wavelength tunable graphene/hBN-based IR photodetectors.
Graphene, an atomically thin carbon sheet with a two-dimensional hexagonal lattice structure, has attracted attention because of its unique electronic and optical properties. Graphene has two promising optical applications: graphene photodetectors that can operate at various wavelengths, and resistive graphene sheets whose optical constants can be configured via an applied voltage. In particular, graphene is a candidate for plasmonic metamaterial absorbers and emitters because of its electrical tunability. We previously demonstrated the concept of multilayer graphene-based metasurfaces. In the present study, we developed a more accurate theoretical calculation model for graphene, and theoretically investigated graphene nanoribbon metasurface absorbers (GNRMAs) for single- or multi-band infrared detection. The GNRMAs consist of a top periodic graphene nanoribbon layer formed on a dielectric layer, which contains another periodic graphene nanoribbon layer and is formed on a back-reflector. High wavelength-selective absorption can be achieved because of the surface plasmon resonance (SPR) of the graphene nanoribbons and Fabry– Pérot resonance of the dielectric layer. The wavelength of graphene SPR is determined mainly by the width of the graphene nanoribbon. The absorption wavelength can be electrically tuned via the chemical potential of graphene, which can be controlled by the voltage applied to the graphene nanoribbons. The gap between the graphene nanoribbon layers determines whether the absorption is single-band or multi-band. This electrical tunability can be enhanced by independently controlling the voltage applied to each graphene nanoribbon layer. These results will contribute to the development of electrically tunable graphene-based multispectral infrared detectors and emitters.
Graphene is a promising material for various optical and electrical device applications because of its high carrier mobility, broadband photoresponse, and low manufacturing cost. One such application is for infrared (IR) photodetectors (PDs) because conventional quantum-type IR PDs require complex and toxic materials such as HgCdTe and Type II superlattice structures. We have developed high-performance graphene IR PDs, which operate in the middle-wavelength or long-wavelength IR (MWIR or LWIR) regions, based on field-effect transistors (FETs) that use a photogating effect. This effect is induced by photosensitizers located around the graphene to produce a voltage change under incident light, inducing a change in the electric current of the graphene, which is attributed to its high carrier mobility and single-atom thickness. Si, InSb, and LiNbO3 were used as the photosensitizers for the visible to near-IR, MWIR, and LWIR, respectively. The photoresponsivity obtained for each wavelength region was more than 10 times greater than that of conventional PDs. However, graphene FET-based structures inevitably produce a large dark current and require three electrical ports, which significantly degenerates the PD performance, inhibiting the use of readout integrated circuits for the IR image sensors. To address this issue, we have developed graphene photogated diodes (GPDs) with graphene/semiconductor heterojunction structures. The GPDs employ Schottky barrier lowering and carrier density modulation by photogating and have recently realized low dark currents and high responsivities because of the graphene/semiconductor Schottky junction and photogating. These results can contribute to the development of high-performance graphene-based IR image sensors.
There is a growing interest in low-cost small-format infrared array sensors. In this study, we demonstrate the properties of small-format graphene infrared array sensors. The devices consisted of 9 x 9 pixels, which were composed of graphene field-effect transistors (FETs) and graphene/semiconductor Schottky barrier diodes (SBDs). The photoresponses of these devices were evaluated under middle-wavelength infrared (MWIR) light irradiation. The graphene FETs exhibited ultrahigh responsivity owing to modulation of the field-effect and surface carriers caused by photocarriers generated in photosensitizers. The MWIR photoresponse of the graphene FETs was enhanced by photogating. Compared to the FETs, the SBDs showed improved dark current characteristics. The photocarriers injected into the graphene were amplified by the photogating effect induced in the graphene/insulator region. Line-scan MWIR images and profiles were obtained; the devices were mounted in ceramic image sensor packages and vacuum-cooled. They were then exposed to a scanning blackbody light source, and the MWIR photoresponse was evaluated. The photocurrent linearly increased with the step shift of the blackbody source. The results obtained in this study will contribute to the development of high-performance graphene-based IR image sensors.
Graphene—atomically thin carbon sheets with a two-dimensional hexagonal lattice structure—exhibits unusual electronic and optical properties. Photodetectors are a good prospective application of graphene because they should ideally exhibit a broadband photoresponse from the ultraviolet to terahertz regions and high-speed operation, as well as be inexpensive to produce. Numerous methods have been proposed in order to enhance the responsivity of graphene-based photodetectors. Among these methods, photogating is most promising because it can realize the highest performance. Photogating requires photosensitive layers at the vicinity of graphene in order to produce a voltage change. Various photosensitive layers, including quantum dots, Si, InSb, and LiNbO3, are used in the visible to near-, mid-, and long-wavelength IR (NIR, MWIR, and LWIR) regions, respectively. However, the operating wavelength region is determined by the photosensitive layer, which undermines the advantage of broadband operation of the graphene. In this work, graphene nanoribbon (GNR) was used as a photosensitive layer. Graphene transistors were prepared using Si substrates with a SiO2 layer and source and drain electrodes. Single-layer graphene fabricated by chemical vapor deposition was transferred onto this substrate and formed a channel, and GNR was formed on the graphene channel using a solution dispersion method. The photoresponse was measured in the mid- and long-wavelength infrared regions. The photoresponse was found to be enhanced by GNR photogating compared with the photoresponse of devices without GNR. These results are expected to contribute to the development of high-performance broadband IR photodetectors.
This study investigated the fabrication and performance of highly responsive photodetectors, constructed of turbostratic stacked graphene produced via chemical vapor deposition (CVD) and using the photogating effect. This effect was induced by situating photosensitizers around a graphene channel such that these materials coupled with incident light and generated large electrical changes. The responsivity of such devices correlates with the carrier mobility of the graphene, and so improved mobility is critical. This work assessed the feasibility of using turbostratic stacked CVD graphene to improve mobility since, theoretically, multilayers of this material may exhibit linear band dispersion, similar to monolayer graphene. This form of graphene also exhibits higher carrier mobility and greater conductivity than monolayer CVD graphene. The turbostratic stacking can be accomplished simply by the repeated transfer of graphene monolayers produced by CVD. Furthermore, it is relatively easy to fabricate CVD graphene layers having sizes suitable for the mass production of electronic devices. Unwanted carrier scattering that can be caused by the substrate is also suppressed by the lower graphene layers when turbostratic stacked graphene is applied. The infrared response properties of the multilayer devices fabricated in the present work were found to be approximately tripled compared with those of a monolayer graphene photodetector. It is evident that turbostratic stacked CVD graphene, which can be produced on a large scale, serves to increase the responsivity of photodetectors in which it is included. The results of this study are expected to contribute to the realization of low-cost, mass-producible, high-responsivity, graphene-based infrared sensors.
Infrared (IR) rectification is promising for high-performance IR detection at room temperature. We propose metal– insulator–metal (MIM)-based plasmonic structures incorporating a nanoslit for IR rectification. Gold and SiO2 were used as the metal and insulator layers, respectively. A high-aspect-ratio nanoslit was incorporated onto the top of the metal layer of an MIM structure. This slit works as a coupler for incident IR light, and a surface plasmon mode is induced in the slit. The coupled IR light is then guided into the middle insulator layer and waveguide modes are formed. Rectification can be achieved by applying a voltage between the top and bottom metal layers. Finite-difference timedomain calculations show that wavelength selective detection can be achieved by controlling the slit width or depth. However, these proposed structures are difficult to fabricate because a metal-based high-aspect-ratio nanoslit cannot be formed by conventional dry or wet etching. We have developed fabrication procedures using gold electroplating and chemical mechanical polishing (CMP). The former method uses a photoresist as a sacrificial layer for the narrow slit, and the top metal is formed by electroplating. The latter uses SiO2 as a sacrificial layer, and the top metal is formed by sputtering and CMP. Both methods can be used to fabricate an MIM structure with a nanoslit. It was found that the CMP method can achieve a higher aspect ratio. These proposed structures and fabrication techniques could contribute to the development of novel IR detectors using plasmonic rectification.
The photoresponse mechanism of graphene/InSb heterojunction middle-wavelength infrared (MWIR) photodetectors was investigated. The devices comprised a graphene/InSb heterojunction as a carrier-injection region and an insulator region of graphene on tetraethyl orthosilicate (TEOS) for photogating. The MWIR photoresponse was significantly amplified with an increase in the graphene/TEOS cross-sectional area by covering the entire detector with graphene. The graphene-channel dependence of the MWIR photoresponse indicated that the graphene carrier density was modulated by photocarrier accumulation at the TEOS/InSb boundary, resulting in photogating. The dark current of the devices was suppressed by a decrease in the graphene/InSb carrier-injection region and the formation of the heterojunction using an n-type InSb substrate. The results indicate that photocarrier transportation was dominated by the formation of a Schottky barrier at the interface of the graphene/InSb heterojunction and a Fermi-level shift under bias application. The high-responsivity and low-dark-current photoresponse mechanism was attributed to the graphene/InSb heterojunction diode behavior and the photogating effect. The devices combining the aforementioned features had a noise equivalent power of 0.43 pW / Hz1/2. The results obtained in our study will contribute to the development of high-performance graphene-based IR image sensors.
Graphene has unique optoelectronic properties and potential applications in improved infrared (IR) photodetectors. Due to its Dirac cone structure, graphene exhibits broadband light absorption and rapid responsivity. In addition, unlike conventional quantum photomaterials, graphene can be synthesized inexpensively via a non-toxic process. Although graphene has advantages in IR photodetector applications, graphene photodetectors have shown low responsivity due to their minimal IR absorption (just 2.3%) and also require cooling. Therefore, there is considerable interest in enhancing the responsivity of graphene photodetectors operating at room temperature so that their advantages can be employed in IR applications. The present work demonstrates room temperature, high-responsivity, long-wavelength infrared (LWIR) graphene photodetectors. These devices operate on the photogating effect, using a lithium niobate (LiNbO3) substrate with enhancement of the photogating via a pyroelectric effect in the substrate in conjunction with a SiN layer. This effect significantly modulates the back-gate voltage to increase the photoresponse by a factor of approximately 600 compared to that for a conventional graphene photodetector. This work also found a change in the type of charge carrier with variations in temperature, which was attributed to a large shift in the Dirac point owing to the strong photogating effect. The results of this study are expected to contribute to the future realization of high-responsivity, low-cost LWIR photodetectors for applications such as thermal imaging, medical care and gas analysis.
Graphene, an atomically thin carbon sheet, has drawn significant attention in many fields because of its unique electronic and optical properties. Graphene is a potential candidate for plasmonic metamaterial absorbers and emitters because of its optical tunability and extreme thinness. We have previously demonstrated graphene Salisbury screen metasurfaces. Although the absorption wavelength of such metasurfaces can be controlled by varying the graphene patch size, the absorbance is insufficient for practical applications. In this study, therefore, multilayer graphene metamaterial absorbers (MGMAs) were theoretically investigated in the middle- to long-wavelength infrared (IR) region. The MGMAs consist of graphene layers alternating with insulator layers formed on a bottom reflector. The spectral absorbance was calculated using the rigorous coupled-wave analysis method. The calculation results demonstrated that a high absorption of ~100% can be achieved because of the multiple plasmonic resonance between each graphene layer and the bottom reflector. The absorption wavelength can be controlled by regulating the graphene pattern size because of the plasmonic resonance of graphene. Furthermore, the absorption wavelength can be tuned by controlling the chemical potential of graphene, which allows for the development of electrically tunable wavelength-selective IR absorbers and emitters. These results will contribute to the development of high-performance wavelength-tunable graphene-based IR detectors and emitters.
Advanced uncooled infrared (IR) sensors with wavelength- or polarization-selectivity are advantageous in applications such as fire detection, gas analysis, hazardous material recognition, biological analysis, and polarimetric imaging. Plasmonic metasurfaces (PM) are potential candidates to realize such functionality over a wide range of wavelength, i.e., middle- to long-wavelength IR regions. In particular, metal-insulator-metal (MIM) PMs are most suitable for image sensor applications because of their small pixel size and simple surface configuration. However, such PMs produce various absorption modes, of which some degrade the wavelength or polarization selectivity. In this study, therefore, control of the absorption modes of PMs was investigated for wavelength- or polarization-selective uncooled IR sensor applications. The PMs produce various absorption modes, namely, localized surface plasmon resonance (LSPR), waveguide, Fabry-Pérot, and plasmonic surface lattice resonance (PSLR) modes. These modes can be controlled by engineering the surface configuration or optimizing the dielectric material. The LSPR mode is appropriate for wide-angle detection because it does not depend on the incident angle, whereas the PSLR mode is a potential candidate for narrowband uncooled IR sensors. The waveguide mode, which can degrade the wavelength and polarization selectivity, can be eliminated using an appropriate dielectric material, such as SiO2 or SiN, for high-performance wavelength- or polarization-selective uncooled IR sensors.
Graphene infrared (IR) photodetectors are promising devices that take advantage of the unique optoelectronic properties of graphene, such as broadband light absorption, rapid response, and high chemical stability. Despite its advantages, graphene has a low absorbance of 2.3%, which limits its photoresponsivity. We have previously reported the responsivity enhancement of graphene middle wavelength IR (MWIR) photodetectors using the photogating effect. The photogating effect is induced by photosensitizers located around the graphene channel that generate a large electrical change. The MWIR photoresponse with the photogating effect was enhanced by 100-fold relative to conventional graphene field-effect transistors (FETs). Although our graphene FETs using photogating exhibited ultrahigh responsivity, the dark current was extremely high, as in the case of conventional graphene FETs, because the normally-OFF operation cannot be realized in graphene. Therefore, reducing the high dark current is essential for applying graphene photodetectors to IR applications. We demonstrate dark current reduction and high responsivity MWIR light detection in graphene MWIR photodetectors. The devices consist of graphene FETs with a carrier injection region. The dark current is reduced by applying a bias voltage. The photocarriers injected into the graphene are amplified by the photogating effect induced in the graphene/insulator region. The dark current of the devices was significantly suppressed compared with that of conventional graphene FETs. The photoresponse characteristics were investigated for devices of different structure sizes. The results obtained in this study will contribute to the development of high-performance graphene-based IR image sensors.
Disorderly stacked multilayer graphene, called turbostratic graphene, is a promising candidate for highly responsive infrared detectors due to its higher carrier mobility than well-ordered multilayer graphene, and facility to suppress the Coulomb scattering from the substrate. Such properties are expected to enhance photogating for high-responsivity infrared detection. The electronic structure of turbostratic graphene was investigated using first-principles calculations. The turbostratic graphene was modeled by introducing disorder to bilayer graphene in terms of the distance and the rotation angle between the graphene layers. The calculation results show that an increase in these parameters leads to linear band dispersion and a structure similar to monolayer graphene.
We demonstrated a middle-wavelength infrared (MWIR) graphene photodetector using the photogating effect. This effect was induced by photosensitizers situated around a graphene channel that coupled incident light and generated a large electrical charge. The graphene-based MWIR photodetector consisted of a top graphene channel, source–drain electrodes, an insulator layer, and a photosensitizer, and its photoresponse characteristics were determined by current measurements. Irradiation of the graphene channel of the vacuum cooled device by an MWIR laser generated a clear photoresponse, as evidenced by modulation of the output current during irradiation. The MWIR photoresponse with the photogating effect was 100 times greater than that obtained from conventional graphene photodetectors without the photogating effect. The device maintained its MWIR photoresponse at temperatures up to 150 K. The effect of the graphene channel size on the responsivity was evaluated to assess the feasibility of reducing the photodetector area, and decreasing the channel area from 100 to 25 μm2 improved the responsivity from 61.7 to 321.0 AW − 1. The results obtained in our study will contribute to the development of high-performance graphene-based IR imaging sensors.
Graphene-based transistors were investigated as simple photodetectors for a broad range of wavelengths. Graphene transistors were prepared using p-doped silicon (Si) substrates with a SiO2 layer, and source and drain electrodes. Monolayer graphene was fabricated by chemical vapor deposition and transferred onto the substrates, and the graphene channel region was then formed. The photoresponse was measured in the broadband wavelength range from the visible, near-infrared (NIR), and mid- to long-wavelength IR (MWIR to LWIR) regions. The photoresponse was enhanced by the photogating induced by the Si substrate at visible wavelengths. Enhancement by the thermal effect of the insulator layer became dominant in the LWIR region, which indicates that the photoresponse of graphene-based transistors can be controlled by the surrounding materials, depending on the operation wavelength. These results are expected to contribute to provide the key mechanism of high-performance graphene-based photodetectors.
Advanced functional infrared (IR) photodetectors with wavelength selectivity are promising for a wide range of applications, such as multicolor imaging, gas analysis and biomedical analysis. Graphene is considered to be a promising material for novel IR detectors. However, the absorption of graphene is constant at approximately 2.3% and rather small. We have developed multispectral high-performance graphene IR photodetectors using metal-insulator-metal (MIM) or single-layer (SL) plasmonic metasurfaces (PMs). MIM- or SL-PMs induce localized surface plasmons on their surfaces and enhance absorption at the wavelength, which can be controlled by their surface patterns, such as the period or the gaps between micropatches. The absorption of graphene with PMs was theoretically investigated for various structural parameters. The absorption wavelength can be controlled based on plasmonic resonance by varying the surface geometry of the PMs. Graphene-based IR photodetectors with SL-PMs were fabricated by the chemical vapor deposition of graphene and then transferred onto the PMs. Wavelength-selective enhancement of the optical absorption and detection by graphene could be achieved due to the effect of the PMs. The results obtained here are expected to contribute to the realization of multispectral graphene infrared image sensors.
Graphene is an atomically thin carbon sheet with a two-dimensional hexagonal lattice structure that has drawn significant attention in many fields due to its unique electronic and optical properties. In this study, graphene Salisbury screen metasurfaces (GSMs) were theoretically investigated as wavelength-selective plasmonic metamaterial absorbers. The GSMs consist of a top graphene sheet, a middle insulator layer and a bottom reflector. The absorption wavelengths of GSMs with a continuous graphene sheet are demonstrated to be controllable according to the insulator layer thickness, which is similar to the case for a conventional Salisbury screen. The insulator thickness can be used to control the optical impedance to incident light using the graphene as a resistive sheet. GSMs with a periodic micropatch array of graphene can be used to control the absorption wavelength, mainly based on the graphene micropatch size and symmetry in conjunction with the insulator thickness. This wavelength selectivity is mainly attributed to the plasmonic resonance in graphene. In both structures, the chemical potential of graphene can be used to tune the absorbance and the absorption wavelength. These results will contribute to the development of electrically tunable and high-performance graphenebased wavelength- or polarization-selective absorbers or emitters.
Graphene has remarkable optoelectronic properties and thus would represent a means to improve infrared (IR) photodetectors. As a result of its Dirac-cone structure, graphene exhibits broadband light absorption and a rapid response. Unlike quantum photomaterials, graphene can also be synthesized inexpensively via a non-toxic process. Despite these advantages, graphene-based photodetectors suffer from low responsivity due to the low absorption of graphene of around 2.3%. Therefore, there is a strong demand to enhance the IR responsivity of graphene photodetectors and expand the range of IR applications. In this study, enhancement of the middle-wavelength IR (MWIR) photoresponsivity of graphene photodetectors using the photogating effect was investigated. The photo-gating effect is induced by photosensitizers, which are located around the graphene channel and couple incident light and generate a large electrical change. The graphenebased MWIR photodetectors consisted of a top graphene channel, source-drain electrodes, insulator layer, and photosensitizer. The photoresponse characteristics were investigated through current measurements using a device analyzer. The device was vacuum-cooled and the graphene channel was irradiated with light from a MWIR laser. The device exhibited a clear MWIR photoresponse observed as modulation of the output current during irradiation. The MWIR photoresponse with the photo-gating effect was 100 times higher than that of conventional graphene photodetectors without the photo-gating effect. The device maintained its MWIR photoresponse at temperatures up to 150 K. The results obtained in this study will contribute to the development of high-performance graphene-based IR image sensors.
Graphene, which is carbon arranged in atomically thin sheets, has drawn significant attention in many fields due to its unique electronic and optical properties. Photodetectors are particularly strong candidates for graphene applications due to the need for a broadband photoresponse from the ultraviolet to terahertz regions, high-speed operation, and low fabrication costs, which have not been achieved with the present technology. Here, graphene-based transistors were investigated as simple photodetectors for a broad range of wavelength. The photoresponse mechanism was determined to be dependent on factors such as the operation wavelength, the components near the graphene channel of the photodetector, and temperature. Here, we report the detailed mechanism that defines the photoresponse of graphene-based transistors. Graphene transistors were prepared using doped silicon (Si) substrates with a SiO2 layer, and source and drain electrodes. Single-layer graphene was fabricated by chemical vapor deposition, transferred onto the substrates, and the graphene channel region was then formed. The photoresponse was measured in the visible, near-infrared (NIR), and mid- and long-wavelength IR (MWIR and LWIR) regions. The results indicated that the photoresponse was enhanced by the Si substrate gating at visible wavelengths. Cooling was required at wavelengths longer than NIR due to thermal noise. Enhancement by the thermal effect of the insulator layer becomes dominant in the LWIR region, which indicates that the photoresponse of graphene-based transistors can be controlled by the surrounding materials, depending on the operation wavelength. These results are expected to contribute to the development of high-performance graphenebased photodetectors.
We describe rare-earth-doped nanophosphors (RE-NPs) for biological imaging using cathodoluminescence (CL) microscopy based on scanning transmission electron microscopy (STEM). We report the first demonstration of multicolor CL nanobioimaging using STEM with nanophosphors. The CL spectra of the synthesized nanophosphors (Y2O3:Eu, Y2O3:Tb) were sufficiently narrow to be distinguished. From CL images of RE-NPs on an elastic carbon-coated copper grid, the spatial resolution was beyond the diffraction limit of light. Y2O3:Tb and Y2O3:Eu RE-NPs showed a remarkable resistance against electron beam exposure even at high acceleration voltage (80 kV) and retained a CL intensity of more than 97% compared with the initial intensity for 1 min. In biological CL imaging with STEM, heavy-metal-stained cell sections containing the RE-NPs were prepared, and both the CL images of RE-NPs and cellular structures, such as mitochondria, were clearly observed from STEM images with high contrast. The cellular CL imaging using RE-NPs also had high spatial resolution even though heavy-metal-stained cells are normally regarded as highly scattering media. Moreover, since the RE-NPs exhibit photoluminescence (PL) excited by UV light, they are useful for multimodal correlative imaging using CL and PL.
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