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Hexagonal boron nitride (hBN) has emerged as a key two-dimensional material. Its importance is linked to that of graphene because it provides an ideal substrate for graphene with minimal lattice mismatch and maintains its high carrier mobility. Moreover, hBN has unique properties in the deep ultraviolet (DUV) and infrared (IR) wavelength bands owing to its indirect bandgap structure and hyperbolic phonon polaritons (HPPs). This review examines the physical properties and applications of hBN-based photonic devices that operate in these bands. A brief background on BN is provided, and the theoretical background of the intrinsic nature of the indirect bandgap structure and HPPs is discussed. Subsequently, the development of DUV-based light-emitting diodes and photodetectors based on hBN’s bandgap in the DUV wavelength band is reviewed. Thereafter, IR absorbers/emitters, hyperlenses, and surface-enhanced IR absorption microscopy applications using HPPs in the IR wavelength band are examined. Finally, future challenges related to hBN fabrication using chemical vapor deposition and techniques for transferring hBN to a substrate are discussed. Emerging techniques to control HPPs are also examined. This review is intended to assist researchers in both industry and academia in the design and development of unique hBN-based photonic devices operating in the DUV and IR wavelength regions.

Graphene has promising applications for novel optoelectronic devices. However, graphene-based photodetectors have two major drawbacks that need attention. The first is how to preserve graphene’s original high carrier mobility, and the second is how to enhance graphene’s absorption to improve its performance. Hexagonal boron nitride (hBN)/graphene van der Waals (vdW) heterostructure-based plasmonic metasurfaces (PMs) are proposed for wavelength-selective infrared (IR) photodetectors. hBN preserves graphene’s high carrier mobility, and PMs enhance graphene’s absorption. Numerical calculations demonstrate sufficient wavelength-selective absorption in the broadband IR wavelength range. Such optical properties are realized by coupling the localized surface plasmon resonance (SPR) of PMs and propagating SPR of graphene. The proposed vdW heterostructure-based PMs could be used for high-performance multi-spectral IR photodetectors.

Hexagonal boron nitride (hBN) exhibits natural hyperbolic dispersion in the infrared (IR) wavelength spectrum. In particular, the hybridization of its hyperbolic phonon polaritons (HPPs) and surface plasmon resonances (SPRs) induced by metallic nanostructures is expected to serve as a new platform for novel light manipulation. In this study, the transmission properties of embedded hBN in metallic one-dimensional (1D) nanoslits were theoretically investigated using a rigorous coupled wave analysis method. Extraordinary optical transmission (EOT) was observed in the type-II Reststrahlen band, which was attributed to the hybridization of HPPs in hBN and SPRs in 1D nanoslits. The calculated electric field distributions indicated that the unique Fabry–Pérot-like resonance was induced by the hybridization of HPPs and SPRs in an embedded hBN cavity. The trajectory of the confined light was a zigzag owing to the hyperbolicity of hBN, and its resonance number depended primarily on the aspect ratio of the 1D nanoslit. Such an EOT is also independent of the slit width and incident angle of light. These findings can not only assist in the development of improved strategies for the extreme confinement of IR light but may also be applied to ultrathin optical filters, advanced photodetectors, and optical devices.

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.

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.

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.

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 presents high-responsivity graphene-based deep-ultraviolet (DUV) photodetectors using chemical vapor deposition (CVD)-hexagonal boron nitride (h-BN) photogating. To improve the DUV photoresponse, h-BN was used as a photosensitizer in graphene field-effect transistors (GFETs). The h-BN photosensitizers were synthesized using CVD and then transferred onto a SiO2/Si substrate. The behavior of h-BN irradiated with DUV light was investigated using cathodoluminescence and UV–VIS reflectance. Under 260 nm light, it exhibited a clear photoresponse with an ultrahigh responsivity of 19600 AW-1, which was 460% higher than a GFET device without h-BN photosensitizers. A noise equivalent power of 3.09×10−13 W/Hz1/2 was achieved.

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.

Surface plasmon polaritons (SPPs) can be generated in graphene at frequencies in the mid-infrared to terahertz range, which is not possible using conventional plasmonic materials such as noble metals. Moreover, the lifetime and confinement volume of such SPPs are much longer and smaller, respectively, than those in metals. For these reasons, graphene plasmonics has potential applications in novel plasmonic sensors and various concepts have been proposed. This review paper examines the potential of such graphene plasmonics with regard to the development of novel high-performance sensors. The theoretical background is summarized and the intrinsic nature of graphene plasmons, interactions between graphene and SPPs induced by metallic nanostructures and the electrical control of SPPs by adjusting the Fermi level of graphene are discussed. Subsequently, the development of optical sensors, biological sensors and important components such as absorbers/emitters and reconfigurable optical mirrors for use in new sensor systems are reviewed. Finally, future challenges related to the fabrication of graphene-based devices as well as various advanced optical devices incorporating other two-dimensional materials are examined. This review is intended to assist researchers in both industry and academia in the design and development of novel sensors based on graphene plasmonics.

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.

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.

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 employ turbostratic stacked chemical vapor deposition (CVD) graphene for a mid-wavelength infrared (MWIR) photodetector using the photogating effect. Turbostratic stacked CVD graphene was fabricated by multiple transfer processes. Graphene field effect transistor-based MWIR photodetectors were developed using an InSb substrate. The effect of the three layers of turbostratic stacked graphene enhanced both the field-effect mobility and MWIR response by approximately three times, compared to that of a conventional single-layer graphene photodetector in vacuum at 77 K. Our results may contribute to the realization of low-cost, mass-producible, high-responsivity graphene-based infrared sensors.

Low dark current, high-responsivity middle-wavelength infrared (IR) graphene photodetectors using photo-gating amplification of injected photo-carriers are demonstrated. A graphene/p-indium antimonide (InSb) heterojunction and graphene/insulator region were formed. The injected photo-carriers from InSb to graphene were amplified by photo-gating induced in the graphene/tetraethyl orthosilicate (TEOS) region, resulting in the high responsivity and low dark current performance. A responsivity of 14.9 A/W and an ON/OFF ratio of 2.66×104 were achieved. The photoresponse is shown to be determined by the cross-sectional area between the graphene and the TEOS-SiO2, in which the injected photo-carriers into graphene were modulated and amplified by the photo-gating effect. Our results indicate that high-performance IR photodetectors based on the developed graphene photodetectors can be realized.

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.

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/semiconductor heterojunction photodetectors have the potential to outperform conventional infrared (IR) sensors. A high-performance graphene/indium antimonide (InSb) heterojunction photodetector for high-resolution mid-IR image sensors was developed using an undoped InSb substrate with low carrier density. The width of the depletion layer at the graphene/InSb interface was increased because of the low carrier density of undoped InSb, which resulted in a low dark current for the proposed device. In addition, the low carrier density resulted in an increased capacitance change due to photocarrier generation for the depletion layer upon light irradiation. As a result, the carrier density modulation effect of graphene was also amplified owing to the photogating effect. Consequently, low dark currents on the order of nanoamperes and high responsivities of over 2 A/W were achieved over a wide voltage range of −0.05 to −0.5 V using our proposed photodetector. The best performance achieved for the developed photodetector corresponded to a specific detectivity (D*) of 2.28 × 1010 cm Hz1/2/W and a noise-equivalent-power of 0.13 pW/Hz1/2, which makes these detectors suitable for use in mid-IR image sensors, realizing a low dark current of just −7.5 nA over the wide wavelength range of 3–5 μm. These results demonstrate that an IR image sensor with detection performance better than those developed using existing graphene-based photodetectors can be obtained by adopting a simple graphene/InSb heterojunction with undoped InSb, owing to the photogating effect.

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, 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.

Metal-insulator-metal-based plasmonic metamaterial absorbers (MIM-PMAs) generate strong localized surface plasmon resonance (LSPR) on their surfaces. Therefore, MIM-PMAs are expected to enhance the absorption of graphene coated on their surfaces. Graphene-coated MIM-PMAs (GMIM-PMAs) were developed and their optical properties were investigated both experimentally and numerically at infrared wavelengths. Significant modification of the absorption of GMIM-PMAs was achieved only in the main LSPR wavelength region, where the insulator is lossless. The enhancement of the absorption of graphene could be maximized by the optimization of the insulator thickness of the MIM-PMAs. The results obtained here are expected to contribute to the development of high-responsivity graphene-based photodetectors and optoelectronic devices.

We present a phosphor nanoparticle that shows both upconversion luminescence (UCL) and cathodoluminescence (CL). With this particle, low-autofluorescence, deep-tissue and wide-field fluorescence imaging can be achieved with nanometer-order high-spatial-resolution imaging. We synthesized Y 2 O 3 :Tm,Yb nanophosphors that emit visible and near-infrared UCL under 980 nm irradiation and blue CL via electron beam excitation. The phosphors were applied to fluorescent imaging of HeLa cells. The photostability of the phosphors was superior to that of a conventional organic dye. We show that after uptake by HeLa cells, the particles can be imaged with SEM and CL contrast in a cellular section. This indicates that correlative UCL and CL imaging of biological samples could be realized.

High-responsivity graphene photodetectors were fabricated using turbostratic stacked graphene, which provided enhanced photogating. Photogating is a promising means of increasing the responsivity of graphene photodetectors, and this effect is proportional to carrier mobility. Turbostratic stacked graphene exhibits higher carrier mobility than conventional monolayer graphene because it has the same band structure as monolayer graphene while preventing scattering by the underlying SiO2 layer. The photoresponse of these devices at a wavelength of 642 nm was approximately twice that obtained for a conventional monolayer graphene photodetector. The results reported show the feasibility of producing high-responsivity graphene-based photodetectors using a simple fabrication technique.

This report describes high-responsivity, long-wavelength infrared graphene photodetectors operating at room temperature, that are based on the photogating effect. Photogating is enhanced by a pyroelectric effect in the lithium niobium oxide (LiNbO3) substrate due to heat generation as a result of radiation absorption by a SiN layer on the substrate. This significantly modulates the back-gate voltage, and increases the photoresponse by a factor of approximately 600. Switching of the charge carrier type in the graphene is observed in response to higher light intensities. The pronounced modulation of the photogating voltage changes the carrier type of graphene.

Comprehensive imaging of a biological individual can be achieved by utilizing the variation in spatial resolution, the scale of cathodoluminescence (CL), and near-infrared (NIR), as favored by imaging probe Gd2O3 co-doped lanthanide nanophosphors (NPPs). A series of Gd2O3:Ln3+/Yb3+ (Ln3+: Tm3+, Ho3+, Er3+) NPPs with multispectral emission are prepared by the sol-gel method. The NPPs show a wide range of emissions spanning from the visible to the NIR region under 980 nm excitation. The dependence of the upconverting (UC)/downconverting (DC) emission intensity on the dopant ratio is investigated. The optimum ratios of dopants obtained for emissions in the NIR regions at 810 nm, 1200 nm, and 1530 nm are applied to produce nanoparticles by the homogeneous precipitation (HP) method. The nanoparticles produced from the HP method are used to investigate the dual NIR and CL imaging modalities. The results indicate the possibility of using Gd2O3 co-doped Ln3+/Yb3+ (Ln3+: Tm3+, Ho3+, Er3+) in correlation with NIR and CL imaging. The use of Gd2O3 promises an extension of the object dimension to the whole-body level by employing magnetic resonance imaging (MRI).

Correlative light and electron microscopy (CLEM) is one attractive method of observing biological specimens because it combines the advantages of both light microscopy (LM) and electron microscopy (EM). In LM, specimens are fully hydrated, and molecular species are distinguished based on the fluorescence colors of probes. EM provides both high-spatial-resolution images superior to those obtained with LM and ultrastructural information of cellular components. The combination of LM and EM gives much more information than either method alone, which helps us to analyze cellular function in more detail.We propose a Y2O3:Tm,Yb phosphor nanoparticle which allows upconversion luminescence (UCL) imaging with near-infrared (NIR) light excitation and cathodoluminescence (CL) imaging [1], where the light emission induced by an electron beam is called cathodoluminescence (CL). Due to electron beam excitation, the spatial resolution of CL microscopy is on the order of nanometers [2,3]. Upconversion is a process in which lower energy, longer wavelength excitation light is transduced to higher energy, shorter wavelength emission light. So far, in LM observation for CLEM, ultraviolet (UV) or visible light has been used for excitation. However, UV and visible light have limited ability to observe deep tissue regions due to absorption, scattering, and autofluorescence. On the other hand, NIR light does not suffer from these problems. Rare-earth-doped upconversion nanophosphors have been applied to biological imaging because of the advantages of NIR excitation [4].We investigated the UCL and CL spectra of Y2O3:Tm,Yb nanophosphors. Y2O3:Tm,Yb nanophosphors that emit visible and near-infrared UCL under 980nm irradiation and blue CL via electron beam excitation. To confirm bimodality of our nanophosphors, correlative UCL/CL images of the nanophosphors were obtained for the same region. The nanophosphors were poured onto a P doped Si substrate (Fig. 1(a)) and were irradiated with 980 nm NIR CW laser light or an electron beam. Fig. 1(b) shows the UCL image of the nanophosphors under 980 nm NIR CW laser irradiation, UCL spots were observed, but the individual nanophosphors in each spot were difficult to distinguish in the UCL image. On the other hand, the edges and the gap between the nanophosphors were clearly distinguished in the CL image (Fig. 1(c)), showing that the spatial-resolution of CL imaging was enough higher than that of UCL image. We believe that upconversion phosphors of the type described here will allow the realization of new CLEM imaging techniques covering the nanometer to millimeter scale, i.e., the molecular to in vivo scale.jmicro;63/suppl_1/i29/DFU073F1F1DFU073F1Fig. 1.(a) SEM and correlative (b) UCL (intensity of 980 nm NIR CW laser 8 mW) and (c) CL images of Y2O3:Tm,Yb nanophosphors in same region (accelerating voltage 3 kV, exposure time 100 ms/pixel).

A variety of bioimaging techniques is regarded as indispensable tools for clinical diagnostics and scientific research. Each technique has its own merits and limitations in terms of spatial resolution, sensitivity, anatomical/functional information and depth. Hence, an interest in development of multimodal imaging, based on hardware or probes platform, is rapidly rising. The combination is expected to provide an comprehensive image incorporating molecular, functional and anatomical information.

Cathodoluminescence (CL) microscopy has a potential to visualize and distinguish specific kinds of biomole-cules at higher spatial resolution than fluorescence microscopy imaging. Up to now, we have investigated multi-color CL observation with different kinds of rare-earth doped nanophosphors (NPs) [1, 2]. The previous results showed that small nanophosphors emit weak CL even though a few tens nm size of particles is indispensable for practical application. Therefore, the improvement of size and CL emission intensity are highly desired. We report the synthesis of highly emissive rare-earth doped NPs smaller than 50 nm and its CL imaging properties.

Correlative light and electron microscopy (CLEM) is one attractive method of observing biological specimens because it combines the advantages of both light microscopy (LM) and electron microscopy (EM) [1]. In LM, specimens are fully hydrated, and molecular species are distinguished by using probes of different colors. EM provides both high spatial resolution images superior to those obtained with LM and highly detailed structural information of cellular components. The combination of LM and EM gives much more information than either method alone, which helps us to analyze cellular function in more detail. However, it is still difficult to distinguish the molecular species with EM images. Quantum dots (QDs) provide information about the biomolecular species in not only LM but also EM [2]. In this method, cells are treated with immuno-staining using QDs, and the target proteins are distinguished by the size of the QDs in EM.