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Thermoelectrically Driven Photocurrent Generation in Femtosecond Laser Patterned Graphene Junctions
Abstract
Single and few-layer graphene photodetectors have attracted much attention in the past few years. Pristine graphene shows a very weak response to visible light, hence fabrication of complex graphene based detectors is a challenging task. In this work, we utilize the ultrafast laser functionalization of single-layer CVD graphene for highly desirable maskless fabrication of micro- and nanoscale devices. We investigate the optoelectronic response of pristine and functionalized devices under femtosecond and continuous wave lasers irradiation. We demonstrate that the photocurrent generation in p-p⁺ junctions formed in single layer graphene is related to the photo-thermoelectric effect. The photoresponsivity of our laser patterned single-layer graphene junctions is shown to be as high as 100 mA/W with noise equivalent power less than 6 kW/cm². These results open a path to a low-cost maskless technology for fabrication of graphene based optoelectronic devices with tunable properties for spectroscopy, signal processing and other applications.
... The recently developed laser-induced two-photon oxidation (TPO) method has enabled precise control of the oxidation conditions in single-and few-layered graphene, offering potential applications in graphene-based electronic and optoelectronic devices using an all-optical approach. [101][102][103][104] This method relies on two-photon processes in ultrafast laser oxidation, which have been studied by nonlinear spectroscopy and imaging [105] and experiments at various wavelengths, [101,103,106] indicating the interactions between light, carbon lattice, oxygen, and water molecules. This method was initially developed during the oxidation of single-walled carbon nanotubes [107] and was later applied to fabricate highly sensitive visible-light detectors. ...
... [198,199] Recent demonstrations of photoluminescence in laser-patterned graphene and MoS 2 highlight the versatility of UDLW in controlling the optical properties of 2D materials. Ultrafast laser technologies play a pivotal role in enhancing the performance of electrical and optoelectronic devices, such as field-effect [17,101,187] and bipolar [200] transistors and photodetectors, [106,122,197] as well as providing band alignment engineering for advanced functionalities. [180] The application of UDLW extends to the development of flexible photodetectors, [201] gas sensors, [202,203] and 3D structures from 2D films, [85,204] demonstrating its broad impact on diverse fields of current research and technology development. ...
... The opening of the bandgap in graphene FETs was demonstrated by local TPO. [101] Utilizing local photochemical doping in a part of the channel in graphene FETs, in-plane junctions such as pn [128] and p-p + [106] have been successfully demonstrated (Figure 9a). The generation of a photocurrent at low power densities in the p-p + junctions formed in single-layer graphene was attributed to the photothermoelectric effect. ...
Ultrafast laser processing has emerged as a versatile technique for modifying materials and introducing novel functionalities. Over the past decade, this method has demonstrated remarkable advantages in the manipulation of 2D layered materials, including synthesis, structuring, functionalization, and local patterning. Unlike continuous‐wave and long‐pulsed optical methods, ultrafast lasers offer a solution for thermal heating issues. Nonlinear interactions between ultrafast laser pulses and the atomic lattice of 2D materials substantially influence their chemical and physical properties. This paper highlights the transformative role of ultrafast laser pulses in maskless green technology, enabling subtractive, and additive processes that unveil ways for advanced devices. Utilizing the synergetic effect between the energy states within the atomic layers and ultrafast laser irradiation, it is feasible to achieve unprecedented resolutions down to several nanometers. Recent advancements are discussed in functionalization, doping, atomic reconstruction, phase transformation, and 2D and 3D micro‐ and nanopatterning. A forward‐looking perspective on a wide array of applications of 2D materials, along with device fabrication featuring novel physical and chemical properties through direct ultrafast laser writing, is also provided.
... This change is caused by TPO of the carbon surface and is irreversible. [30,32] The fast charge thermalization leads to the generation of hot carriers [33] upon fs-laser pulses in the presence of oxidative species. [20] It plays a major role in carbon lattice engineering during TPO. ...
... The defect density can be locally altered up to 10 12 cm −2 with a preserved integrity of a hexagonal lattice of a nanotube. These changes coincide with our previous results for graphene functionalization via fs-laser pulses, [28,32] where we demonstrated that the majority of the defects are oxygen groups, especially epoxy groups, which were grafted to the surface via TPO. Note that during TPO the oxidative groups tend to organize clusters rather than scatter [29] that can increase the local defect density near the planar junction. ...
... 2021, 7, 2000872 www.advelectronicmat.de the photo-thermoelectric effect, originating from different Seebeck coefficients in pristine and modified parts of a nanotube, as was previously shown for graphene photodetector. [32] The second one is the photovoltaic effect arising from the formation of an intramolecular planar junction between the modified and pristine part of SWCNT, which is responsible for a nonlinear behavior of the output I-V curves. The parasitic electrostatic gating by the trapped charges in the substrate near the modified nanotube could also affect the generated charges under the light. ...
The fabrication of planar junctions in carbon nanomaterials is a promising way to increase the optical sensitivity of optoelectronic nanometer‐scale devices in photonic connections, sensors, and photovoltaics. Utilizing a unique lithography approach based on direct femtosecond laser processing, a fast and easy technique for modification of single‐walled carbon nanotube (SWCNT) optoelectronic properties through localized two‐photon oxidation is developed. It results in a novel approach of quasimetallic to semiconducting nanotube conversion so that metal/semiconductor planar junction is formed via local laser patterning. The fabricated planar junction in the field‐effect transistors based on individual SWCNT drastically increases the photoresponse of such devices. The broadband photoresponsivity of the two‐photon oxidized structures reaches the value of 2 × 107 A W−1 per single SWCNT at 1 V bias voltage. The SWCNT‐based transistors with induced metal/semiconductor planar junction can be applied to detect extremely small light intensities with high spatial resolution in photovoltaics, integrated circuits, and telecommunication applications. A low‐cost, facile, and versatile direct patterning technique based on femtosecond laser processing is reported. This method is applied to individual single‐walled carbon nanotube transistors to convert quasimetallic to semiconducting nanotubes by grafting oxygen species and form a planar junction between pristine and modified parts of a nanotube to detect ultralow light intensities in a broadband light range.
... By combining laser processing techniques with the unique optical properties of 2D materials, various laser-assisted 2D material photodetectors have been explored to realize high performance and multifunctional nano-optoelectronic devices. [140][141][142] First, we introduce several important parameters that are used to determine the detection performance of the photodetectors, including photoresponsivity (R), photoresponse time (rise time t r and fall time t f ) and noise equivalent power (NEP). ...
... Recently, Emelianov et al. utilized an ultrafast laser (515 nm) to modify a monolayer graphene-based photodetector to increase the photoresponse of the device. 141 Fig. 13d displays a schematic of the device and band alignment of the graphene FFET (GFET) under laser irradiation. Local fs laser modification of the graphene channel can trigger doping effects, resulting in diverse junction generation, such as p-p + , n-n À and p-n. ...
Two dimensional (2D) materials have generated enormous interest in various research fields in recent years due to their fascinating properties, such as atomic thickness, distinct structures and superior characteristics. In particular, electronic and optoelectronic devices based on 2D materials have been widely investigated, which is promising for innovation of future nanoelectronics technology. Recently, the technique of laser processing 2D materials with the advantages of facility, versatility and low cost has been reported to enable the fabrication of diverse 2D material electronic and optoelectronic devices, such as field-effect transistors (FETs), p-n junction diodes, bipolar junction transistors (BJTs), inverters, memories, photodetectors, photovoltaic devices and light-emitting devices. Here, various interactions between lasers and 2D materials are introduced, such as direct writing processing, reduction, doping, phase transition engineering, thinning and oxidation. The current advances in laser processing-assisted 2D material electronic and optoelectronic devices are reviewed. A comprehensive outlook on the future development of 2D material nanoelectronics /optoelectronics via laser processing is also presented.
... 10,[13][14][15] Typically, graphene based PTE PDs have been studied widely, such as single-bilayer interface junction formation, antenna array structure construction, and preparation of reduced graphene oxide. 12,16,17 In our recent work, we have reported the PTE photoresponses of three-dimensional graphene foam (3D GF) 3 and reduced graphene oxide/CsPbrBr 3 . 18 However, limited by the complex structure and slow response time, it is still hardly to be used widely for graphene based PTE PDs. ...
... 26 In general, the most effective strategy is to construct heterojunction using two materials with different Seebeck coefficients to improve the overall PTE response. 16,27 Polymer based PTE PDs such as poly (3,4ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT:PSS) devices are receiving increasing attentions because of their high thermoelectric properties, solution processing capability and high flexibility. [27][28][29][30] The high Seebeck coefficient of up to 436 μV/K 30 and high electrical conductivity of up to ∼10 4 S/m of PEDOT:PSS 28 can be beneficial for electrons transmission in PTE devices. ...
Organic–inorganic halide perovskite with low thermal conductivity, high Seebeck coefficient and high carrier mobility are promising thermoelectric materials for near infrared (NIR) and terahertz (THz) photodetector (PD). Here, we report a novel rapid response and self-powered NIR and THz photothermoelectric PD based on CH3NH3PbI3 (MAPbI3) and poly (3,4-ethylenedioxythiophene): poly (4-styrenesulfonate) (PEDOT:PSS) composite. An order of magnitude enhancement in the Seebeck coefficient was observed that resulted from the addition of PEDOT:PSS. Under 1064 nm and 2.52 THz illumination, the device displays stable and repeatable photoresponse at room temperature under a zero bias voltage. The frequency response shows a -3dB frequency band of 5 kHz, corresponding to a fast response time of 28 μs, which is approximately three orders of magnitude faster than previously reported results. These results demonstrate that MAPbI3 /PEDOT:PSS is a promising composite material for fast response and self-powered NIR-THz PTE PD operating at room temperature.
... Two-photon oxidation (2PO) is an optical method of functionalizing and patterning graphene with ultrashort laser pulses. 41 It allows a highly localized, controllable, and finetunable way of introducing oxygen-containing hydrophilic groups 42 onto graphene, thereby modifying its electrical properties 41,43 and interactions with its surroundings. 44,45 The photoinduced chemical groups mainly consist of epoxide (C-O-C) and hydroxyl (-OH) groups. ...
Femtosecond pulsed laser two-photon oxidation (2PO) was used to modulate protein adsorption on graphene surfaces on a Si/SiO2 substrate. The adsorption behavior of calmodulin (CaM) and a muscarinic acetylcholine receptor (mAchR) fragment on pristine (Pr) and 2PO-treated graphene were studied, utilizing atomic force microscopy and infrared scattering-type scanning near-field optical microscopy for characterization. The results showed that proteins predominantly bound as a (sub-)monolayer, and selective adsorption could be achieved by carefully varying graphene oxidation level, pH during functionalization, and protein concentration. The most pronounced selectivity was observed at low 2PO levels, where predominantly only point-like oxidized defects are generated. Preferential binding on either Pr or oxidized graphene could be achieved depending on the 2PO and adsorption conditions used. Based on the incubation conditions, the surface area covered by mAchR on single-layer graphene varied from 29% (Pr) vs. 91% (2PO) to 48% (Pr) vs. 13% (2PO). For CaM, the coverage varied from 53% (Pr) vs. 95% (2PO) to 71% (Pr) vs. 52% (2PO). These results can be exploited in graphene biosensor applications via selective non-covalent functionalization of sensors with receptor proteins.
... By using a femtosecond laser to functionalize a singlelayer graphene, a p-p+ junction was constructed and a high Seebeck coefficient difference was realized. [161] As a basic structure strategy of photodetectors, a gate-coupled architecture was applied in many photodetectors to enhance the photoresponse of the device. An electric field formed by applying an external gate voltage can modulate the carrier concentration in the channel of the photodetectors, and further modulate the Seebeck coefficient. ...
2D materials, with outstanding optical, thermal, and electric properties, are emerging as promising candidates for fabricating high‐performance photodetectors. Recently, impressive progresses have been made in this area and some challenges are remaining to improve the properties of photodetectors. As one important part in the mainstream photodetection mechanisms, photothermoelectric (PTE) effect is showing unique priorities in fabricating advanced photodetectors, especially broadband detection operating in the mid‐infrared and terahertz spectral regime. Here, recent progress on PTE photodetectors based on layered 2D materials is reviewed. The physical mechanism of PTE effect is first discussed and then the optical and thermoelectric properties of various 2D materials are analyzed. Furthermore, strategies to improve the photodetection performance of PTE detectors are summarized in two major categories including enhanced photothermal conversion and thermoelectric conversion processes. Finally, the challenges and prospects for future research in 2D thermoelectric materials and PTE detectors are also provided.
... The laser parameters for the 2PO of graphene used in this work were optimized (to achieve the highest ID/IG ratio) as reported previously. [19,20] The oxidation was verified by Raman spectroscopy ( Figure S1, Supporting Information). ...
Biosensors based on graphene and bio‐graphene interfaces have gained momentum in recent years due to graphene's outstanding electronic and mechanical properties. By introducing the patterning of a single‐layer graphene surface by two‐photon oxidation (2PO), the surface hydrophobicity/hydrophilicity and doping can be varied at the nanoscale while preserving the carbon network, thus opening possibilities to design new devices. In this study, the effect of 2PO on the catalytic activity of the noncovalently immobilized enzyme horseradish peroxidase (HRP) on single‐layer graphene‐coated Si/SiO 2 chips is presented. To monitor the activity continuously, a simple well‐plate setup is introduced. Upon controllable 1–2‐layer immobilization, the catalytic activity decreases to a maximum value of 7.5% of the free enzyme. Interestingly, the activity decreases with increasing 2PO area on the samples. Hence, the HRP catalytic activity on the graphene surface is locally controlled. This approach can enable the development of graphene‐bio interfaces with locally varying enzyme activity.
... Thus, the metal oxide could be deposited on graphene with properties tuned from conductive to nearly insulating. [25,30] Higher laser irradiation doses led to a more uniform, smoother layer deposition due to a higher density of active sites. Moreover, we show that graphene properties could be restored after ALD treatment using thermal annealing, without altering the deposited metal oxide layer. ...
Area-selective atomic layer deposition (ALD) is a promising “bottom-up” alternative to current nanopatterning techniques. While it has been success-fully implemented in traditional microelectronic processes, selective nuclea-tion of ALD on 2D materials has so far remained an unsolved challenge. In this article, a precise control of the selective deposition of ZnO on graphene at low temperatures (<250 °C) is demonstrated. Maskless femtosecond laser writing is used to locally activate predefined surface areas (down to 300 nm) by functionalizing graphene to achieve excellent ALD selectivity (up to 100%) in these regions for 6-nm-thick ZnO films. The intrinsic conductive proper-ties of graphene can be restored by thermal annealing at low temperature (300 °C) without destroying the deposited ZnO patterns. As the graphene layer can be transferred onto other material surfaces, the present patterning technique opens new attractive ways for various applications in which the functionalized graphene is utilized as a template layer for selective deposition of desired materials.
... The 4-inch silicon wafers were used to produce chips (52 chips per wafer), each containing an array of GFETs using the high-throughput transfer technique described in (Emelianov et al., 2018;Kireev et al., 2016). In brief, the single-layer graphene was transferred onto a Si substrate with 300 nm SiO 2 layer by a wet transfer and then patterned to form graphene channels via oxygen plasma etching (300 W, 200 sccm, 10 min). ...
Mycotoxins comprise a frequent type of toxins present in food and feed. The problem of mycotoxin contamination has been recently aggravated due to the increased complexity of the farm-to-fork chains, resulting in negative effects on human and animal health and, consequently, economics. The easy-to-use, on-site, on-demand, and rapid monitoring of mycotoxins in food/feed is highly desired. In this work, we report on an advanced mycotoxin biosensor based on an array of graphene field-effect transistors integrated on a single silicon chip. A specifically designed aptamer against Ochratoxin A (OTA) was used as a recognition element, where it was covalently attached to graphene surface via pyrenebutanoic acid, succinimidyl ester (PBASE) chemistry. Namely, an electric field stimulation was used to promote more efficient π-π stacking of PBASE to graphene. The specific G-rich aptamer strand suggest its π-π stacking on graphene in free-standing regime and reconfiguration in G-quadruplex during binding an OTA molecule. This realistic behavior of the aptamer is sensitive to the ionic strength of the analyte solution, demonstrating a 10-fold increase in sensitivity at low ionic strengths. The graphene-aptamer sensors reported here demonstrate fast assay with the lowest detection limit of 1.4 pM for OTA within a response time as low as 10 s, which is more than 30 times faster compared to any other reported aptamer-based methods for mycotoxin detection. The sensors hold comparable performance when operated in real-time within a complex matrix of wine without additional time-consuming pre-treatment.
... Their later work revealed that the application of the resulting graphene patterns in p-p + junctions exhibited a high photoresponsivity of 100 mA W −1 . [101] ...
The realization that nanostructured graphene featuring nanoscale width can confine electrons to open its bandgap has aroused scientists’ attention to the regulation of graphene structures, where the concept of graphene patterns emerged. Exploring various effective methods for creating graphene patterns has led to the birth of a new field termed graphene patterning, which has evolved into the most vigorous and intriguing branch of graphene research during the past decade. The efforts in this field have resulted in the development of numerous strategies to structure graphene, affording a variety of graphene patterns with tailored shapes and sizes. The established patterning approaches combined with graphene chemistry yields a novel chemical patterning route via molecular engineering, which opens up a new era in graphene research. In this review, the currently developed graphene patterning strategies is systematically outlined, with emphasis on the chemical patterning. In addition to introducing the basic concepts and the important progress of traditional methods, which are generally categorized into top‐down, bottom‐up technologies, an exhaustive review of established protocols for emerging chemical patterning is presented. At the end, an outlook for future development and challenges is proposed. Graphene patterning has attracted widespread attention owing to its ability to tailor the structures and properties of graphene, targeting high‐tech applications. The established strategies have been evolved from the initial dimension regulation to the emerging molecular engineering of graphene sheet. In general, these methods can be divided into top‐down/bottom‐up and chemical patterning routes as comprehensively disused in this review.
... The 4-inch silicon wafers were used to produce chips (52 chips per wafer), each containing an array of GFETs using the high-throughput transfer technique described in [20,21]. In brief, the single-layer graphene was transferred onto a Si substrate with 300 nm SiO2 layer by a wet transfer and then patterned to form graphene channels via oxygen plasma etching (300 W, 200 sccm, 10 min). ...
Mycotoxins comprise a frequent type of toxins present in food and feed. The problem of mycotoxin contamination has been recently aggravated due to the increased complexity of the farm-to-fork chains, resulting in negative effects on human and animal health and, consequently, economics. The easy-to-use, on-site, on-demand, and rapid monitoring of mycotoxins in food/feed is highly desired. In this work, we report on an advanced bioelectronic mycotoxin sensor based on graphene field-effect transistors integrated on a silicon chip. A specific aptamer for Ochratoxin A (OTA) was attached to graphene through covalent bonding with the pyrene-based linker, which was deposited with an electric field stimulation to increase the surface coverage. This graphene/aptamer sensor demonstrates high sensitivity to OTA with the lowest detection limit of 1.4 pM within a response time of 10 s which is superior to any other reported aptamer-based methods.
... The effectiveness of this strategy has been examined through comprehensive experimental and theoretical calculations in metal-oxide materials. [8][9][10] By contrast, the dynamic modification of flowing PC has been successfully achieved using thermal, [11,12] force, [13] and electric fields. [14,15] For dynamic modification, studies have focused on the noncontact approach, in which a magnetic field is used as an external driving force. ...
The rarely explored, spin-polarized band engineering, enables direct dynamic control of the magneto-optical absorption (MOA) and associated magneto-photocurrent (MPC) by a magnetic field, greatly enhancing the range of applicability of photosensitive semiconductor materials. It is demonstrated that large negative and positive MOA and MPC effects can be tuned alternately in amorphous carbon (
a-C
)/ZnO nanowires by controlling the sp2/sp3 ratio of
a-C
. A sizeable enhancement of the MPC ratio (≈15%) appears at a relatively low magnetic field (≈0.2 T). Simulated two peaks spin-polarized density of states is applied to explain that the alternate sign switching of the MOA is mainly related to the charge transfer between ZnO and C. The results indicate that the enhanced magnetic field performance of (
a-C
)/ZnO nanowires may have applications in renewable energy-related fields and tunable magneto-photonics.
... Such a system of extraordinary temperature gradients customizable with laser powers, can be exploited as important platforms for a wide range of nanoscale thermodynamics investigations, such as the heat transport across a nanoscale distance and the thermoelectrics of nanomaterials. [30][31][32][33] Precision reduction opens opportunities for custom-designing the properties of GO lms, such as tuning the electrical conductivity, optical band gap and WF. Here we demonstrate that the precisely controlled reduction can be used to ne tune GO's WF. ...
Being able to precisely control the reduction of two-dimensional graphene oxide films will open exciting opportunities for tailor-making the functionality of nanodevices with on-demand properties. Here we report meticulously controlled reduction of individual graphene oxide flakes ranging from single to seven layers through controlled laser irradiation. It is found that the reduction can be customized in such a precise way that the film thickness can be accurately thinned with sub-nanometer resolution, facilitated by extraordinary temperature gradients >102 K/nm across the interlayers of graphene oxide films. Such precisely controlled reduction provides important pathways towards precision nanotechnology with custom-designed electrical, thermal, optical and chemical properties. We demonstrate that this can be exploited to finely tune the work function of graphene oxide films with unprecedented precision of only a few milli electronvolts.
... The energy gap in gapless graphene can be open, for example, by adsorbing hydrogen on its surface [9] or depositing graphene on a hexagonal boron nitride substrate [10]. It was shown in [11] that oxidation of graphene by femtosecond laser irradiation leads to the formation of a planar graphenegraphene oxide heterostructures (see, e.g., [12]). ...
A planar graphene-based heterostructure is considered, which behaves differently: as a barrier or a quantum well at small or large momenta of charge carriers, respectively. This heterostructure contains a strip of gapped graphene with a lower Fermi velocity, surrounded by gapless graphene with a higher Fermi velocity. In this configuration, an interface state arises at the intersection point of dispersion curves. The transformation of this interface state into a fundamental bound state is investigated.
Self-powered and flexible ultrabroadband photodetectors (PDs) are desirable in a wide range of applications. The current PDs based on the photothermoelectric (PTE) effect have realized broadband photodetection. However, most of them express low photoresponse and lack of flexibility. In this work, high-performance, self-powered, and flexible PTE PDs based on laser-scribed reduced graphene oxide (LSG)/CsPbBr3 are developed. The comparison experiment with LSG PD and fundamental electric properties show that the LSG/CsPbBr3 device exhibits enhanced ultrabroadband photodetection performance covering ultraviolet to terahertz range with high photoresponsivity of 100 mA/W for 405 nm and 10 mA/W for 118 μm at zero bias voltage, respectively. A response time of 18 ms and flexible experiment are also acquired at room temperature. Moreover, the PTE effect is fully discussed in the LSG/CsPbBr3 device. This work demonstrates that LSG/CsPbBr3 is a promising candidate for the construction of high-performance, flexible, and self-powered ultrabroadband PDs at room temperature.
Photothermoelectric (PTE) effect can effectively convert light into electricity through photothermal and thermoelectric processes, and has great potential applications in light energy harvesting and bandgap-independent photodetection. It is particularly applicable for the terahertz (THz) range featuring low photon energy, but has not been well established due to lack of high-performance PTE materials in this range. Three-dimensional microporous graphene (3DMG) foam possesses ultrahigh THz absorptivity and outstanding photothermal conversion, and can serve as a promising candidate. Here, enhancement of the THz PTE response of 3DMG foam by fine tuning its thermoelectric properties using the ionic liquid electric double layer (EDL) technique was demonstrated. Continuous and reversible control of the Seebeck coefficient of 3DMG highlights the effectiveness of EDL gating in manipulating the electronic structures of such bulk and porous material. An approximate one order of magnitude enhancement in the Seebeck coefficient as well as the PTE responsivity was observed. In addition, a double-cell 3DMG EDL device with a p-n junction like channel configuration enabled further improvement of the photoresponse. This work opens a new avenue to optimize the PTE performance of 2D nanosheet-assembled 3D porous materials for highly efficient energy harvesting and detection of THz radiation.
Highly sensitive broadband photodetection is of critical importance for many applications. However, it is a great challenge to realize broadband photodetection by using a single device. Here we report photodetectors (PDs) based on three-dimensional (3D) graphene foam (GF) photodiodes with asymmetric electrodes, which show an ultra-broadband photoresponse from ultraviolet to microwave for wavelengths ranging from 102 to 106 nm. Moreover, the devices exhibit a high photoresponsivity of 103 A ·W−1, short response time of 43 ms, and 3 dB bandwidth of 80 Hz. The high performance of the devices can be attributed to the photothermoelectric (PTE, also known as the Seebeck) effect in 3D GF photodiodes. The excellent optical, thermal, and electrical properties of 3D GFs offer a superior basis for the fabrication of PTE-based PDs. This work paves the way to realize ultra-broadband and high-sensitivity PDs operated at room temperature.
In this work, we report a novel method of maskless doping of a graphene channel in a field-effect transistor configuration by local inkjet printing of organic semiconducting molecules. The graphene-based transistor was fabricated via large-scale technology, allowing for upscaling electronic device fabrication and lowering the device’s cost. The altering of the functionalization of graphene was performed through local inkjet printing of N,N′-Dihexyl-3,4,9,10-perylenedicarboximide (PDI-C6) semiconducting molecules’ ink. We demonstrated the high resolution (about 50 µm) and accurate printing of organic ink on bare chemical vapor deposited (CVD) graphene. PDI-C6 forms nanocrystals onto the graphene’s surface and transfers charges via π–π stacking to graphene. While the doping from organic molecules was compensated by oxygen molecules under normal conditions, we demonstrated the photoinduced current generation at the PDI-C6/graphene junction with ambient light, a 470 nm diode, and 532 nm laser sources. The local (in the scale of 1 µm) photoresponse of 0.5 A/W was demonstrated at a low laser power density. The methods we developed open the way for local functionalization of an on-chip array of graphene by inkjet printing of different semiconducting organic molecules for photonics and electronics.
This work is focused on the fabrication and analysis of graphene-based, solution-gated field effect transistor arrays (GFETs) on a large scale for bioelectronic measurements. The GFETs fabricated on different substrates, with a variety of gate geometries (width/length) of the graphene channel, reveal a linear relation between the transconductance and the width/length ratio. The area normalised electrolyte-gated transconductance is in the range of 1–2 mS·V⁻¹·□ and does not strongly depend on the substrate. Influence of the ionic strength on the transistor performance is also investigated. Double contacts are found to decrease the effective resistance and the transfer length, but do not improve the transconductance. An electrochemical annealing/cleaning effect is investigated and proposed to originate from the out-of-plane gate leakage current. The devices are used as a proof-of-concept for bioelectronic sensors, recording external potentials from both: ex vivo heart tissue and in vitro cardiomyocyte-like HL-1 cells. The recordings show distinguishable action potentials with a signal to noise ratio over 14 from ex vivo tissue and over 6 from the cardiac-like cell line in vitro. Furthermore, in vitro neuronal signals are recorded by the graphene transistors with distinguishable bursting for the first time.
Graphene-based photodetectors have demonstrated mechanical flexibility, large operating bandwidth, and broadband spectral response. However, their linear dynamic range (LDR) is limited by graphene’s intrinsic hot-carrier dynamics, which causes deviation from a linear photoresponse at low incident powers. At the same time, multiplication of hot carriers causes the photoactive region to be smeared over distances of a few micrometers, limiting the use of graphene in high-resolution applications. We present a novel method for engineering photoactive junctions in FeCl3-intercalated graphene using laser irradiation. Photocurrent measured at these planar junctions shows an extraordinary linear response with an LDR value at least 4500 times larger than that of other graphene devices (44 dB) while maintaining high stability against environmental contamination without the need for encapsulation. The observed photoresponse is purely photovoltaic, demonstrating complete quenching of hot-carrier effects. These results pave the way toward the design of ultrathin photodetectors with unprecedented LDR for high-definition imaging and sensing.
Ultrafast and broad spectral bandwidth photodetectors are desirable attributable to their unique bandstructures. Photodetectors based on graphene have great potential due to graphene’s outstanding optical and electrical properties. However, the highest reported values of the photoresponsivity of pure graphene are less than 10 mA/W at room temperature, which significantly limits its potential applications. Here, we report a photoresponsivity of 32 A/W in pure monolayer graphene photodetectors, an improvement of over one order of magnitude for functional graphene nanostructures (<3 A/W). The high photocurrent generation in our devices can be attributed to the high sensitivity of graphene’s resistivity to a local change of the electric field induced by photo-excited carriers generated in the light-doping substrate. This dramatically increases the feasibility of using graphene for the next generation of photodetectors.
Recording extracellular potentials from electrogenic cells (especially neurons) is the hallmark destination of modern bioelectronics. While fabrication of flexible and biocompatible in vivo devices via silicon technology is complicated and time-consuming, graphene field-effect transistors (GFETs), instead, can easily be fabricated on flexible and biocompatible substrates. In this work, we compare GFETs fabricated on rigid (SiO2/Si and sapphire) and flexible (polyimide) substrates. The GFETs, fabricated on the polyimide exhibit extremely large transconductance values, up to 11 mS·V-1, and mobility over 1750 cm2·V-1·s-1. In vitro recordings from cardiomyocyte-like cell culture are performed by GFETs on a rigid transparent substrate (sapphire). Via multichannel measurement we are able to record and analyze both: difference in action potentials as well as their spatial propagation over the chip. Furthermore, the controllably flexible polyimide-on-steel (PIonS) substrates are able to ex vivo record electrical signals from primary embryonic rat heart tissue. Considering the flexibility of PIonS chips, together with the excellent sensitivity, we open up a new road into graphene-based in vivo biosensing.
The micro-scale patterns in graphene on Si/SiO2 substrate were fabricated using ultrashort 515 nm laser pulses. For both picosecond and femtosecond laser pulses two competitive processes, based on photo-thermal (ablation) and photochemical (oxidation/etching) effects, were observed. The laser-induced etching of graphene starts just below the threshold energy of graphene ablation: 1.7 nJ per 280 fs pulse and 2.7 µJ per 30 ps pulse. Whilst etching is not sensitive to thermal properties of graphene and provides uniform patterns, the ablation, in contrast, is highly affected by defects in the graphene structure like wrinkles and bilayer islands. The mechanisms of ultrafast laser interaction with graphene are discussed.
The doping effect on graphene by photoresists were studied in this article. Polymethyl methacrylate (PMMA) is the usual choice for graphene transfer, but it is known to leave a significant amount of residue. PMMA results in strong hole doping and reduction of mobility of the graphene devices. Not only PMMA, but photoresists also leave residues during the lithographic steps and dope the graphene in strong hole-doping states along with water and oxygen molecules. In this article, we tested three types of photoresists for their effects on graphene's electrical properties. It was found that a specific photoresist can significantly reduce the amount of hole-doping of the graphene transistor more than other photoresists. The use of hydrophobic substrates and additional thermal treatment can help reducing the hole-doping further.
We present a photoresponse study on a lateral defect/pristine graphene junction device fabricated by a simple plasma irradiation method. The junction between pristine graphene and plasma-modified graphene was created by controlling the location of Ar⁺ plasma treatment. We found that a distinct photocurrent was generated at the junction by photocurrent line scanning measurements, and further analysis reveals that the photothermoelectric (PTE) effect, instead of the photovoltaic (PV) effect, dominates the photocurrent generation at the interface. Additionally, the obtained results suggest that tuning the defect density could be effective in modulating the optoelectronic performance of junctions in our device.
Graphene-based photodetectors have recently received much attention for their potential to detect weak signals and their short response time, both of which are crucial in applications such as optical positioning, remote sensing, and biomedical imaging. However, existing devices for detecting weak signals are limited by the current photogating mechanism, so the price for achieving ultrahigh sensitivity is to sacrifice response time. In this work, we bridge the gap between ultrafast response and ultrahigh sensitivity by employing a graphene / SiO 2 / lightly doped Si architecture with an interfacial gating mechanism. Our device is capable of detecting a signal of < 1 nW (with a responsivity of ∼ 1000 AW − 1 ), and the spectral response extends from the visible to near-IR. More important, the photoresponse time of our device has been pushed to ∼ 400 ns . The current device structure does not need a complicated fabrication process and is fully compatible with silicon technology. This work not only will open up a route to graphene-based high-performance optoelectronic devices but also has great potential for ultrafast weak signal detection.
The development rate of graphene-related research is tremendous. New methods of graphene growth and transfer are reported on a regular basis, trending towards large-scale. Nevertheless, the fabrication of high-yield and low-cost graphene devices is still challenging. In this work, we approach this problem from a technological point of view and propose a new, so-called "high-throughput transfer technique". The technique allows a semi-automatic transfer of graphene films right at the desired places on a wafer. We demonstrate the applicability of our method by aligning 52 graphene devices on a 4-inch wafer using only 4 cm2 of graphene. The overall yield of this process is over 90%.
Graphene has drawn tremendous attention as a promising candidate for electronic and optoelectronic applications owing to its extraordinary properties, such as broadband absorption and ultrahigh mobility. Nevertheless, the absence of a bandgap makes graphene unfavorable for digital electronic or photonic applications. Although patterning graphene into nanostructures with the quantum confinement effect is able to open a bandgap, devices based on these graphene nanostructures generally suffer from low carrier mobility and scattering losses. In this paper, we demonstrated that encapsulation of an atomic layer deposited high-quality HfO2 film will greatly enhance the carrier mobility and decrease the scattering losses of graphene nanoribbons, because this high-k dielectric layer weakens carrier coulombic interactions. In addition, a photodetector based on HfO2 layer capped graphene nanoribbons can cover broadband wavelengths from visible to mid-infrared at room temperature, exhibiting ∼10 times higher responsivity than the one without a HfO2 layer in the visible regime and ∼8 times higher responsivity in the mid-infrared regime. The method employed here could be potentially used as a general approach to improve the performance of graphene nanostructures for electronic and optoelectronic applications.
Graphene has been considered as an attractive material for optoelectronic applications such as photodetectors owing to its extraordinary properties, e.g. broadband absorption and ultrahigh mobility. However, challenges still remain in fundamental and practical aspects of the conventional graphene photodetectors which normally rely on the photoconductive mode of operation which has the drawback of e.g. high dark current. Here, we demonstrated the photovoltaic mode operation in graphene p-n junctions fabricated by a simple but effective electron irradiation method that induces n-type doping in intrinsic p-type graphene. The physical mechanism of the junction formation is owing to the substrate gating effect caused by electron irradiation. Photoresponse was obtained for this type of photodetector because the photoexcited electron-hole pairs can be separated in the graphene p-n junction by the built-in potential. The fabricated graphene p-n junction photodetectors exhibit a high detectivity up to ~3 × 10(10) Jones (cm Hz(1/2) W(-1)) at room temperature, which is on a par with that of the traditional III-V photodetectors. The demonstrated novel and simple scheme for obtaining graphene p-n junctions can be used for other optoelectronic devices such as solar cells and be applied to other two dimensional materials based devices.
We present the science and technology roadmap (STR) for graphene, related two-dimensional (2d) crystals, and hybrid systems, targeting an evolution in technology, with impacts and benefits reaching into most areas of society. The roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. In this document we provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlithing the roadmap to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries: from flexible, wearable and transparent electronics to high performance computing and spintronics.
Graphene, being an ultrathin, durable, flexible, transparent material with
superior conductivity and unusual optical properties, promises many novel
applications in electronics, photonics and optoelectronics. For applications in
electronics, patterning and modification of electrical properties is very
desirable since pristine graphene has no band gap. Here we demonstrate a simple
all-optical patterning method for graphene, based on laser induced two-photon
oxidation. By tuning the intensity of irradiation and the number of pulses the
level of oxidation can be controlled to high precision and, therefore, a band
gap can be introduced and electrical and optical properties can be continuously
tuned. Arbitrary complex patterning can be performed for air-suspended
monolayer graphene or for graphene on substrates. The method works at room
temperature in ambient air and no additional processing step is needed. The
presented concept allows development of all-graphene electronic and
optoelectronic devices and complex circuits with an all-optical method.
Energy transfer to acoustic phonons is the dominant low-temperature cooling channel of electrons in a crystal. For cold neutral graphene we find that the weak cooling power of its acoustic modes relative to their heat capacity leads to a power-law decay of the electronic temperature when far from equilibrium. For heavily doped graphene a high electronic temperature is shown to initially decrease linearly with time at a rate proportional to n;{3/2} with n being the electronic density. The temperature at which cooling via optical phonon emission begins to dominate depends on graphene carrier density.
This paper provides an overview on graphene solution-gated field-effect transistors (SGFETs) and their applications in bioelectronics. The fabrication and characterization of arrays of graphene SGFETs is presented and discussed with respect to competing technologies. To obtain a better understanding of the working principle of solution-gated transistors, the graphene-electrolyte interface is discussed in detail. The in vitro biocompatibility of graphene is assessed by primary neuron cultures. Finally, bioelectronic experiments with electrogenic cells are presented, confirming the suitability of graphene to record the electrical activity of cells.
Low-frequency noise with a spectral density that depends inversely on
frequency (f) has been observed in a wide variety of systems including current
fluctuations in resistors, intensity fluctuations in music and signals in human
cognition. In electronics, the phenomenon, which is known as 1/f noise, flicker
noise or excess noise, hampers the operation of numerous devices and circuits,
and can be a significant impediment to development of practical applications
from new materials. Graphene offers unique opportunities for studying 1/f noise
because of its 2D structure and carrier concentration tuneable over a wide
range. The creation of practical graphene-based devices will also depend on our
ability to understand and control the low-frequency 1/f noise in this material
system. Here, I review the characteristic features of 1/f noise in graphene and
few-layer graphene, and examine the implications of such noise for the
development of graphene-based electronics including high-frequency devices and
sensors.
Graphene has attracted large interest in photonic applications owing to its promising optical properties, especially its ability to absorb light over a broad wavelength range, which has lead to several studies on pure monolayer graphene-based photodetectors. However, the maximum responsivity of these photodetectors is below 10 mA W(-1), which significantly limits their potential for applications. Here we report high photoresponsivity (with high photoconductive gain) of 8.61 A W(-1) in pure monolayer graphene photodetectors, about three orders of magnitude higher than those reported in the literature, by introducing electron trapping centres and by creating a bandgap in graphene through band structure engineering. In addition, broadband photoresponse with high photoresponsivity from the visible to the mid-infrared is experimentally demonstrated. To the best of our knowledge, this work demonstrates the broadest photoresponse with high photoresponsivity from pure monolayer graphene photodetectors, proving the potential of graphene as a promising material for efficient optoelectronic devices.
We compute the electronic component (κ) of the thermal conductivity and the thermoelectric power (α) of monolayer graphene within the hydrodynamic regime, taking into account the slow rate of carrier population imbalance relaxation. Interband electron-hole generation and recombination processes are inefficient due to the nondecaying nature of the relativistic energy spectrum. As a result, a population imbalance of the conduction and valence bands [i.e., a nonequilibrium state with μe+μh≠0, where μe (μh) denotes the electron (hole) chemical potential] is generically induced upon the application of a thermal gradient. We show that the thermoelectric response of a graphene monolayer depends upon the ratio of the sample length to an intrinsic length scale lQ set by the imbalance relaxation rate. At the same time, we incorporate the crucial influence of the metallic contacts required for the thermopower measurement (under open circuit boundary conditions) since carrier exchange with the contacts also relaxes the imbalance. These effects are especially pronounced for clean graphene, where the thermoelectric transport is limited exclusively by intercarrier collisions. For specimens shorter than lQ, the population imbalance extends throughout the sample; κ and α asymptote toward their zero imbalance relaxation limits. In the opposite limit of a graphene slab longer than lQ, at nonzero doping κ and α approach intrinsic values characteristic of the infinite imbalance relaxation limit. Samples of intermediate (long) length in the doped (undoped) case are predicted to exhibit an inhomogeneous temperature profile, while κ and α grow linearly with the system size. In all cases except for the shortest devices, we develop a picture of bulk electron and hole number currents that flow between thermally conductive leads, where steady-state recombination and generation processes relax the accumulating imbalance. Our analysis incorporates, in addition, the effects of (weak) quenched disorder.
Graphene is a promising candidate for optoelectronic applications such as
photodetectors, terahertz imagers, and plasmonic devices. The origin of
photoresponse in graphene junctions has been studied extensively and is
attributed to either thermoelectric or photovoltaic effects. In addition, hot
carrier transport and carrier multiplication are thought to play an important
role. Here we report the intrinsic photoresponse in biased but otherwise
homogeneous graphene. In this classic photoconductivity experiment, the
thermoelectric effects are insignificant. Instead, the photovoltaic and a
photo-induced bolometric effect dominate the photoresponse due to hot
photocarrier generation and subsequent lattice heating through electron-phonon
cooling channels respectively. The measured photocurrent displays polarity
reversal as it alternates between these two mechanisms in a backgate voltage
sweep. Our analysis yields elevated electron and phonon temperatures, with the
former an order higher than the latter, confirming that hot electrons drive the
photovoltaic response of homogeneous graphene near the Dirac point.
We fabricate graphene-TiOx-Al tunnel junctions and characterize their radio
frequency response. Below the superconducting critical temperature of Al and
when biased within the superconducting gap, the devices show enhanced dynamic
resistance which increases with decreasing temperature. Application of radio
frequency radiation affects the dynamic resistance through electronic heating.
The relation between the electron temperature rise and the absorbed radiation
power is measured, from which the bolometric parameters, including heat
conductance, noise equivalent power and responsivity, are characterized.
We report on the intrinsic optoelectronic response of high-quality dual-gated monolayer and bilayer graphene p-n junction
devices. Local laser excitation (of wavelength 850 nanometers) at the p-n interface leads to striking six-fold photovoltage
patterns as a function of bottom- and top-gate voltages. These patterns, together with the measured spatial and density dependence
of the photoresponse, provide strong evidence that nonlocal hot carrier transport, rather than the photovoltaic effect, dominates
the intrinsic photoresponse in graphene. This regime, which features a long-lived and spatially distributed hot carrier population,
may offer a path to hot carrier–assisted thermoelectric technologies for efficient solar energy harvesting.
The recent discovery of graphene has led to many advances in two-dimensional physics and devices. The graphene devices fabricated so far have relied on SiO(2) back gating. Electrochemical top gating is widely used for polymer transistors, and has also been successfully applied to carbon nanotubes. Here we demonstrate a top-gated graphene transistor that is able to reach doping levels of up to 5x1013 cm-2, which is much higher than those previously reported. Such high doping levels are possible because the nanometre-thick Debye layer in the solid polymer electrolyte gate provides a much higher gate capacitance than the commonly used SiO(2) back gate, which is usually about 300 nm thick. In situ Raman measurements monitor the doping. The G peak stiffens and sharpens for both electron and hole doping, but the 2D peak shows a different response to holes and electrons. The ratio of the intensities of the G and 2D peaks shows a strong dependence on doping, making it a sensitive parameter to monitor the doping.
We find experimentally that the optical sheet conductance of graphite per graphene layer is very close to (pi/2)e2/h, which is the theoretically expected value of dynamical conductance of isolated monolayer graphene. Our calculations within the Slonczewski-Weiss-McClure model explain well why the interplane hopping leaves the conductance of graphene sheets in graphite almost unchanged for photon energies between 0.1 and 0.6 eV, even though it significantly affects the band structure on the same energy scale. The f-sum rule analysis shows that the large increase of the Drude spectral weight as a function of temperature is at the expense of the removed low-energy optical spectral weight of transitions between hole and electron bands.
Transfer of graphene and other two-dimensional materials is still a technical challenge. The 2D-materials are typically patterned after transfer, which leads to a major loss of material. Here, we present laser induced forward transfer of chemical vapor deposition grown graphene layers with well-defined shapes and geometries. The transfer is based on photo-decomposition of a triazene-based transfer layer that produces N2 gas, which propels a graphene layer from the donor to the acceptor substrate. The functionality of the graphene-metal junction was verified by realizing functional bottom contact bottom gate field-effect transistors.
The possibility of laser induced modification of local mechanical properties of polycrystalline chemical vapor deposition graphene on silicon substrate in air has been demonstrated. Nanosecond laser pulses (wavelength 532 nm) with focal spot diameter ~1 μm were used. Samples were placed and irradiated inside a scanning probe microscope (SPM) that allowed in situ studies of surface morphology and mechanical phase contrast in SPM tapping mode before and after multipulsed laser treatment. It was found that along with local profile transformation of graphene sheet (formation of nanopits and nanobumps), transformation of mechanical properties of graphene on a substrate structure took place. Such laser modified graphene area is larger than (but of the order of) the irradiation spot size. Its appearance is related to laser induced radial extension of an adsorbed water nanolayer intercalated between graphene and substrate. It is shown that the process of water layer lateral migration has a reversible character. This effect is proved by laser spot shift and sequential irradiation.
Control of the type and density of charge carriers in graphene is essential for its implementation into various practical applications. Here, we demonstrate the gate-tunable doping effect of adsorbed piperidine on graphene. By gradually increasing the amount of adsorbed piperidine, the graphene doping level can be varied from p- to n-type, with the formation of p-n junctions for intermediate coverages. Moreover, the doping effect of the piperidine can be further tuned by the application of large negative back-gate voltages, which increase the doping level of graphene. In addition, the electronic properties of graphene are well preserved due to the non-covalent nature of the interaction between piperidine and graphene. Overall, this gate-tunable doping offers an easy, controllable, and non-intrusive method to alter the electronic structure of graphene.
The exceptional optical and electronic properties of graphene make it a promising material for photodetection, especially applications requiring fast and sensitive response to light across the spectrum ranging from the visible to the infrared down to the terahertz domain. However, the ultrashort lifetime of photocarriers caused by the fast recombination of graphene results in the weak response of light and limits its application in photodetection. To overcome the restriction of limited lifetime of photocarriers in photodetection, it is necessary to introduce graphene p-n junctions to generate photocurrent or photovoltage efficiently, and numerous efforts have been made. In this review, we first give an overview of photodetection and then evaluate physical and chemical methods available for the fabrication of graphene p-n junctions. Subsequently, we provide a detailed discussion on current research advances in enhancing the performance of graphene-based photodetectors, mainly focusing on the coupling of graphene with photonic structures and building vertical heterostructures. We believe that the potential commercialization of graphene p-n junction based photodetectors will be promoted by the development on the scalable production of graphene and its integration with highly developed silicon-based photonic and electronic platforms.
Direct laser writing is a technology with excellent prospects for mask-less processing of carbon-based nanomaterials, because of the wide range of photoinduced reactions that can be performed on large surfaces with submicron resolution. In this paper, we demonstrate the use of picoseconds laser pulses for one-step ablation and functionalization of graphene. Varying the parameters of power, pulse frequency, and speed, we demonstrated the ablation down to 2μm width and up to mm-long lines as well as functionalization with spatial resolution less than 1μm with linear speeds in the range of 1m/s. Raman and atomic-force microscopy studies were used to indicate the difference in modified graphene states and correlation to the changes in optical properties.
The graphene-based photodetector with tunable p-p+-p junctions was fabricated through a simple laser irradiation process. Distinct photoresponse was observed at the graphene (G)-laser irradiated
graphene (LIG) junction by scanning photocurrent measurements, and its magnitude can be modulated as a result of a positive correlation between the photocurrent and doping concentration in LIG region. Detailed investigation suggests that the photo-thermoelectric effect, instead of the photovoltaic effect, dominates the photocurrent generation at the G-LIG junctions. Such a simple and low-cost technique offers an alternative way for the fabrication of graphene-based optoelectronic devices.
We report the gate-modulated Raman spectrum of defective graphene. We show that the intensity of the D peak can be reversibly tuned by applying a gate voltage. This effect is attributed to chemical functionalization of the graphene crystal lattice, generated by an electrochemical reaction involving the water layer trapped at the interface between silicon and graphene.
We report the gate-tunable photoresponse of a defective graphene over the ultraviolet (UV) and the visible light illumination, where the defect was generated by plasma irradiation. Plasma induced Dirac point shift indicates the p-doping effect. Interestingly the defective-graphene field effect transistor (defective -GFET) showed a negative shift upon UV illumination, whereas the device showed a positive shift under visible light illumination, along with the change in the photocurrent. The defective -GFET device showed a high photoresponsivity of 37 mAW-1 under visible light, that is ~ 3 times higher than that of the pristine graphene device. Photoinduced molecular desorption causes the UV light responsivity to 18 mAW-1. This study shows that the tunable photodetector with high responsivity is feasible by introducing an artificial defect on graphene surface.
Graphene and other two-dimensional materials, such as transition metal dichalcogenides, have rapidly established themselves as intriguing building blocks for optoelectronic applications, with a strong focus on various photodetection platforms. The versatility of these material systems enables their application in areas including ultrafast and ultrasensitive detection of light in the ultraviolet, visible, infrared and terahertz frequency ranges. These detectors can be integrated with other photonic components based on the same material, as well as with silicon photonic and electronic technologies. Here, we provide an overview and evaluation of state-of-the-art photodetectors based on graphene, other two-dimensional materials, and hybrid systems based on the combination of different two-dimensional crystals or of two-dimensional crystals and other (nano)materials, such as plasmonic nanoparticles, semiconductors, quantum dots, or their integration with (silicon) waveguides.
The dynamics of fs-laser ablation of graphite has been investigated experimentally and theoretically. The experimental observation of two different ablation mechanisms is supported by molecular dynamics calculations, which incorporate the changes of the interatomic potentials due to electronic excitation.
We report on temperature dependent photocurrent measurements of high-quality
dual-gated monolayer graphene (MLG) p-n junction devices. A photothermoelectric
(PTE) effect governs the photocurrent response in our devices, allowing us to
track the hot electron temperature and probe hot electron cooling channels over
a wide temperature range (4 K to 300 K). At high temperatures (), we
found that both the peak photocurrent and the hot spot size decreased with
temperature, while at low temperatures (), we found the opposite,
namely that the peak photocurrent and the hot spot size increased with
temperature. This non-monotonic temperature dependence can be understood as
resulting from the competition between two hot electron cooling pathways: (a)
(intrinsic) momentum-conserving normal collisions (NC) that dominates at low
temperatures and (b) (extrinsic) disorder-assisted supercollisions (SC) that
dominates at high temperatures. Gate control in our high quality samples allows
us to resolve the two processes in the same device for the first time. The peak
temperature depends on carrier density and disorder concentration, thus
allowing for an unprecedented way of controlling graphene's photoresponse.
We report on nanometer-scale patterning of single layer graphene on SiO2/Si substrate through femtosecond laser ablation. The pulse fluence is adjusted around the single-pulse ablation threshold of graphene. It is shown that, even though both SiO2 and Si have more absorption in the linear regime compared to graphene, the substrate can be kept intact during the process. This is achieved by scanning the sample under laser illumination at speeds yielding a few numbers of overlapping pulses at a certain point, thereby effectively shielding the substrate. By adjusting laser fluence and translation speed, 400 nm wide ablation channels could be achieved over 100 μm length. Raster scanning of the sample yields well-ordered periodic structures, provided that sufficient gap is left between channels. Nanoscale patterning of graphene without substrate damage is verified with Scanning Electron Microscope and Raman studies.
This paper reports the formation of uniform single layer micro-patterns of graphene on a glass substrate using direct femtosecond laser cutting. The cutting of graphene was achieved in air and argon. By translating the graphene sample with respect to the laser beam, continuous micro-channels were carved. The cutting geometry can be controlled by varying the laser fluence and the scanning path. Also, 1∼2 µm wide graphene micro-ribbons were hatched out. The ablation threshold of graphene was determined to be 0.16∼0.21 J/cm2. With the laser fluence higher than the ablation threshold, graphene was ablated rapidly and removed completely without damaging the glass substrate. Atomic force microscopy (AFM) and Raman spectroscopy have been used to confirm the ablation of graphene. Time domain finite difference modelling was employed to understand the thermal history of the laser ablation process.
We report the hysteresis induced by two similar defects located at and near the SiO2–Si interface in the Al/CeO2–SiO2/Si capacitor. We find that the hysteresis are generated directly due to the presence of interface trap and anomalous positive charge. Electrical characteristics of the hysteresis are, however, distinct and are strongly dependent on the type of defects. For example, the hysteresis caused by interface traps disappeared after passivating the Si dangling bonds by a post metallization annealing, while only a bias-temperature annealing causes the reduction of the hysteresis generated by anomalous positive charge. We suggest the mechanisms of the hysteresis generation in the Al/CeO2–SiO2/Si capacitor.
Anderson has shown that there is no diffusion of an electron in certain random lattices, and Mott has pointed out that, for electrons in materials in which there is a potential energy varying in a random way from atom to atom, Anderson's work predicts that there should be a range of energies at the bottom of the conduction band for which an electron can move only by thermally activated hopping from one localized state to another. An energy Ec will separate the energies where this happens from the nonlocalized range of energies where there is no thermal activation. Cerium sulfide, investigated some years ago by Cutler and Leavy, is a particularly suitable material testing whether this is so because, in the neighborhood of the composition Ce2S3, 19 of the cerium sites are vacancies distributed at random, and the number of free electrons can be varied with only very small changes in the number of vacancies. It is shown that the experimental results find a natural explanation in terms of this model: Conduction is by hopping when the concentration of electrons is low and the Fermi energy EF lies below Ec; but when the concentration is higher and EF>Ec, conduction is by the usual band mechanism with a short mean free path. The thermoelectric power is examined in both ranges, and the Hall mobility in the hopping region (EF
With its electrical carrier type as well as carrier-densities highly-sensitive to light, graphene is potentially an ideal candidate for many opto-electronic applications. Beyond the direct light-graphene interactions, indirect effects arising from induced charge traps underneath the photoactive graphene arising from light-substrate interactions must be better understood and harnessed. Here, we study the local doping effect in graphene using focused-laser irradiation, which governs the trapping and ejecting behavior of the charge trap-sites in the gate oxide. The local doping effect in graphene is manifested by a large Dirac voltage shifts and/or double Dirac peaks from the electrical measurements and a strong photocurrent response due to the formation of a p-n-p junction in gate-dependent scanning photocurrent microscopy. The technique of focused-laser irradiation on a graphene device suggests a new method to control the charge-carrier type and carrier concentration in graphene in a non-intrusive manner as well as elucidate strong light-substrate interactions in the ultimate performance of graphene devices.
The responsivity of graphene photodetectors depends critically on the elevated temperature of the electronic subsystem upon photoexcitation. We investigate the role of the substrate in providing cooling pathways for photoexcited carriers under ambient conditions by partially suspending few-layer graphene over a trench. Through photocurrent microscopy, we observe p-n junctions near the supported/suspended interfaces that produce photothermoelectric currents. Most importantly, we find the photocurrent in suspended p-n junctions to be an order of magnitude larger than in supported structures. This enhancement is attributed to the elimination of a dominant electronic cooling channel via the surface phonons of the polar substrate. Our work documents this mechanism of energy exchange between graphene and its environment, and it points to the importance of dielectric engineering for future improved graphene photodetectors.
We have examined the interfacial thermal conductance GK of single and multilayer graphene samples prepared on fused SiO2 substrates by mechanical exfoliation of graphite. By using an ultrafast optical pump pulse and monitoring the transient reflectivity on the picosecond time scale, we obtained an average value of GK of GK=5000 W/cm2 K for the graphene/SiO2 interface at room temperature. We observed significant variation in GK between individual samples, but found no systematic dependence on the thickness of the graphene layers.
In the last three decades, zero-dimensional, one-dimensional, and two-dimensional carbon nanomaterials (i.e., fullerenes, carbon nanotubes, and graphene, respectively) have attracted significant attention from the scientific community due to their unique electronic, optical, thermal, mechanical, and chemical properties. While early work showed that these properties could enable high performance in selected applications, issues surrounding structural inhomogeneity and imprecise assembly have impeded robust and reliable implementation of carbon nanomaterials in widespread technologies. However, with recent advances in synthesis, sorting, and assembly techniques, carbon nanomaterials are experiencing renewed interest as the basis of numerous scalable technologies. Here, we present an extensive review of carbon nanomaterials in electronic, optoelectronic, photovoltaic, and sensing devices with a particular focus on the latest examples based on the highest purity samples. Specific attention is devoted to each class of carbon nanomaterial, thereby allowing comparative analysis of the suitability of fullerenes, carbon nanotubes, and graphene for each application area. In this manner, this article will provide guidance to future application developers and also articulate the remaining research challenges confronting this field.
Raman spectroscopy is able to probe disorder in graphene through defect-activated peaks. It is of great interest to link these features to the nature of disorder. Here we present a detailed analysis of the Raman spectra of graphene containing different type of defects. We found that the intensity ratio of the D and D' peak is maximum (∼13) for sp(3)-defects, it decreases for vacancy-like defects (∼7), and it reaches a minimum for boundaries in graphite (∼3.5). This makes Raman Spectroscopy a powerful tool to fully characterize graphene.
Device instabilities of graphene metal-oxide-semiconductor field effect transistors such as hysteresis and Dirac point shifts have been attributed to charge trapping in the underlying substrate, especially in SiO 2 . In this letter, trapping time constants around 87 μ s and 1.76 ms were identified using a short pulse current-voltage method. The values of two trapping time constants with reversible trapping behavior indicate that the hysteretic behaviors of graphene field effect transistors are due to neither charge trapping in the bulk SiO 2 or tunneling into other interfacial materials. Also, it is concluded that the dc measurement method significantly underestimated the performance of graphene devices.
We present the results of a thorough study of wet chemical methods for transferring chemical vapor deposition grown graphene from the metal growth substrate to a device-compatible substrate. On the basis of these results, we have developed a "modified RCA clean" transfer method that has much better control of both contamination and crack formation and does not degrade the quality of the transferred graphene. Using this transfer method, high device yields, up to 97%, with a narrow device performance metrics distribution were achieved. This demonstration addresses an important step toward large-scale graphene-based electronic device applications.
Strong electron-electron interactions in graphene are expected to result in multiple-excitation generation by the absorption of a single photon. We show that the impact of carrier multiplication on photocurrent response is enhanced by very inefficient electron cooling, resulting in an abundance of hot carriers. The hot-carrier-mediated energy transport dominates the photoresponse and manifests itself in quantum efficiencies that can exceed unity, as well as in a characteristic dependence of the photocurrent on gate voltages. The pattern of multiple photocurrent sign changes as a function of gate voltage provides a fingerprint of hot-carrier-dominated transport and carrier multiplication.
We study photodetection in graphene near a local electrostatic gate, which enables active control of the potential landscape and carrier polarity. We find that a strong photoresponse only appears when and where a p-n junction is formed, allowing on-off control of photodetection. Photocurrents generated near p-n junctions do not require biasing and can be realized using submicrometer gates. Locally modulated photoresponse enables a new range of applications for graphene-based photodetectors including, for example, pixilated infrared imaging with control of response on subwavelength dimensions.