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José M. Caridad, Óscar Castelló, Sofía M. López Baptista, [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), Hartmut G. Roskos, Juan A. Delgado-Notario

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[Room-Temperature Plasmon-Assisted Resonant THz Detection in Single-Layer Graphene Transistors](https://mdr.nims.go.jp/datasets/07ddca39-aca0-48b0-b430-a65178027c71)

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Room-Temperature Plasmon-Assisted Resonant THz Detection in Single-Layer Graphene TransistorsRoom-Temperature Plasmon-Assisted Resonant THz Detection inSingle-Layer Graphene TransistorsJosé M. Caridad, Óscar Castelló, Sofía M. López Baptista, Takashi Taniguchi, Kenji Watanabe,Hartmut G. Roskos, and Juan A. Delgado-Notario*Cite This: Nano Lett. 2024, 24, 935−942 Read OnlineACCESS Metrics & More Article Recommendations *sı Supporting InformationABSTRACT: Frequency-selective or even frequency-tunable terahertz (THz) photodevices are critical components for manytechnological applications that require nanoscale manipulation, control, and confinement of light. Within this context, gate-tunablephototransistors based on plasmonic resonances are often regarded as the most promising devices for the frequency-selectivedetection of THz radiation. The exploitation of constructive interference of plasma waves in such detectors promises not onlyfrequency selectivity but also a pronounced sensitivity enhancement at target frequencies. However, clear signatures of plasmon-assisted resonances in THz detectors have been revealed only at cryogenic temperatures so far and remain unobserved at application-relevant room-temperature conditions. In this work, we demonstrate the sought-after room-temperature resonant detection of THzradiation in short-channel gated photodetectors made from high-quality single-layer graphene. The survival of this intriguingresonant regime at room temperature ultimately relies on the weak intrinsic electron−phonon scattering in monolayer graphene,which avoids the damping of the plasma oscillations present in the device channel.KEYWORDS: terahertz, resonant detection, plasmons, graphene, two dimensional materialsTerahertz (THz) radiation (0.1−10 THz) has a strongperspective in a wide range of different applications,including metrology and characterization of nanomaterials,1upcoming 6G wireless communications,2 noninvasive imaging,3biosensing,4 high-resolution spectroscopy,5 together with manyothers.6,7 An emerging and important research area within THztechnology is the study of novel, efficient, and functionalphotodetectors operating at these frequencies.8 The majority ofphotodetectors reported to date (if not all), including sensorsmade of many different nanomaterials,9−15 operate either inbroadband mode (i.e., without being selective to a givenfrequency) at room temperature or over narrow fixed frequencybands (i.e., without being frequency tunable), for example, byembedding antennas in the detector. Frequency-tunable THzphotodetectors working at atmospheric conditions are thereforeunavailable so far, despite being desirable components to (i)boost the performance of some applications at specific andselected THz wavelengths16 and (ii) provide new functionalitiessuch as selective sensing, frequency mixing, multiplication, andmodulation as well as nanoscale confinement of light.17One of the most prominent ideas to design tunable andselective THz photodetectors, originally introduced by M.Dyakonov and M. Shur more than two decades ago, predicts thattwo-dimensional (2D) gated FETs may exhibit a resonantresponse to electromagnetic THz radiation at discrete plasmaoscillation frequencies of the 2D electrons in the devicechannel.17 In this pioneering proposal, the resonant operationof field-effect transistor (FET) photodetectors is univocallydefined by a quality factor, Q = ωτ, which must be much largerthan unity (Q = ωτ ≫ 1, where ω = 2πf with f being theReceived: November 7, 2023Revised: December 22, 2023Accepted: December 27, 2023Published: January 2, 2024Letterpubs.acs.org/NanoLett© 2024 The Authors. Published byAmerican Chemical Society935https://doi.org/10.1021/acs.nanolett.3c04300Nano Lett. 2024, 24, 935−942This article is licensed under CC-BY 4.0Downloaded via NATL INST FOR MATLS SCIENCE (NIMS) on January 26, 2024 at 00:25:35 (UTC).See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Jose%CC%81+M.+Caridad"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="O%CC%81scar+Castello%CC%81"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Sofi%CC%81a+M.+Lo%CC%81pez+Baptista"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takashi+Taniguchi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Kenji+Watanabe"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Hartmut+G.+Roskos"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Hartmut+G.+Roskos"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Juan+A.+Delgado-Notario"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/acs.nanolett.3c04300&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c04300?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c04300?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c04300?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c04300?goto=supporting-info&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c04300?fig=tgr1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c04300?fig=tgr1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c04300?fig=tgr1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c04300?fig=tgr1&ref=pdfhttps://pubs.acs.org/toc/nalefd/24/3?ref=pdfhttps://pubs.acs.org/toc/nalefd/24/3?ref=pdfhttps://pubs.acs.org/toc/nalefd/24/3?ref=pdfhttps://pubs.acs.org/toc/nalefd/24/3?ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://doi.org/10.1021/acs.nanolett.3c04300?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://pubs.acs.org/NanoLett?ref=pdfhttps://pubs.acs.org/NanoLett?ref=pdfhttps://acsopenscience.org/researchers/open-access/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/frequency of the incoming radiation and τ the momentumrelaxing scattering time of charge carriers in the system,respectively). In other words, resonant THz photodetectionshould arise in plasmonic FETs at any temperature, when anegligible damping of the plasma waves occurs in the channel. Insuch conditions, the device channel acts as a tunable plasmoniccavity with a set of multiple resonances defined by the incomingfrequency, the device length, and the density of charge carriers inthe system.17 This exotic regime is in clear contrast to the morecommonly observed and studied broadband (nonresonant oroverdamped) case,18−23 characterized by Q≪ 1, with plasmonsbeing strongly damped in the channel and even decaying longbefore reaching the other side of the plasmonic cavity.To date, several experimental studies have attempted todemonstrate resonant THz detection in different 2D electrongases systems with varying levels of success. Convincingsignatures of plasmon resonances, including the appearance offrequency-dependent oscillations in the zero-bias photores-ponse of the system w.r.t. the carrier density, have beenidentified at cryogenic temperatures in FET devices made ofsome high-quality semiconductors such as III−V materials24−27and bilayer graphene.28 However, such features vanish rapidlywhen operating above cryogenic temperatures and long beforereaching room temperature. This fact notably limits thepotential use of resonant THz photodetectors for real-lifeapplications.6,7In this Letter, we demonstrate room-temperature THzdetection in FET devices made of high-quality, single-layergraphene. In particular, we show how the characteristicfrequency-dependent oscillations in the photoresponse ofmonolayer graphene FETs are largely tunable with the densityof charge carriers in the device (i.e., with the applied top gatevoltage), and these unique fingerprints of the resonant detectionare furthermore visible from cryogenic up to room temperature.The fact that these robust signatures persist up to 300 K in ourdevices can be directly ascribed to the weak acoustic phononscattering in monolayer graphene, which leads to large carriermobility values in the material even at elevated temper-atures.29,30 In other words, as shown below, the resonantcondition Q ≫ 1 is also fulfilled in high-quality single-layergraphene FET detectors at room temperature.In order to observe plasmonic resonant THz detection, wefabricated a short-channel (length Lch = 6 μm) dual-gate, high-mobility, single-layer graphene FET device (Figure 1a) by usingFigure 1. Graphene-based resonant THz photodetector. (a) Optical images of the graphene THz detector with a bow-tie antenna coupled betweensource and top gate electrodes. The bottom image shows a zoomed view of the device with source (S), top gate (TG), and drain (D) electrodes labeled.(b) Schematic 3D view of the zero-bias photocurrent measurements of the device. (c) Current responsivity, RI, as a function of the top gate voltage,VTG, measured at 0.3 THz. The upper-right inset shows the current responsivity expected from the DC conductivity, following the phenomenologicalformula21 ΔI= −dσ/dVTG. The bottom-left inset shows the evolution of theQ factor in our device as a function of the excitation frequency. The dashedline corresponds to Q = 1. Highlighted areas in blue and red indicate the frequencies ranges in which our device operates the overdamped or weaklydamped regimes, respectively. (d) RI as a function of the top gate voltage measured at 4.7 THz. Inset panels show zoomed in areas of the recordedcurrent responsivity for electron (upper-right) and hole (bottom-left) carriers. Responsivity resonances are highlighted by red arrows in these insets.Here, we note that if the current responsivity is calculated as RI = IPCST/PSD, where ST is the THz beam spot area and SD is the detector active area, thedevice performance would reach larger maximum values of ∼0.29 A/W at 0.3 THz and ∼1.8 mA/W at 4.7 THz.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.3c04300Nano Lett. 2024, 24, 935−942936https://pubs.acs.org/doi/10.1021/acs.nanolett.3c04300?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c04300?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c04300?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c04300?fig=fig1&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.3c04300?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asa state-of-the-art dry-stacking technique22 to encapsulate amechanically exfoliated single-layer graphene sheet in betweentwo thin hexagonal boron nitride (hBN) flakes. The graphenewas then side-contacted to Cr/Au (3.5/50 nm) metallicelectrodes acting as drain and source contacts. In addition, ametal top-gate electrode covering most of the FET channel (LTG= 4.8 μm) was defined on the device, together with a coupledbow tie antenna between top-gate and source electrodes. Thisantenna ensures an efficient rectification of the incoming THzradiation for a large range of frequencies via gate-to-sourcecoupling (additional fabrication details are shown in SupportingInformation, Note 1).Transport and zero-bias photocurrent measurements in ourdevice were performed in a closed-cycle cryostat, with thechamber temperature varying from 10 K up to 300 K. Weemployed two different THz sources to perform the photo-current experiments. First, a sub-THz source was used toundertake measurements at a frequency of 0.3 THz, and then, aquantum cascade laser was used to undertake measurements atfrequencies in a range between 2.5 THz up to 4.7 THz (moreinformation about the photocurrent setup can be found in refs21 and 22).First, we measured the transport characteristics of ourgraphene FET via electrical measurements from 10 to 300 K(see Supporting Information Note 2). We extracted averagemobilities, μ, in the device exceeding 70000 cm2 V−1 s−1 for bothelectron and hole carriers at low temperatures (10 K). Suchvalues remain high, above 60000 cm2 V−1 s−1, even at roomtemperature (Supporting Information Note 2 contains themeasured electrical data as well as details to calculate the carriermobility). We further estimated the momentum-relaxingscattering time of charge carriers in the device τ to lie between0.29 and 0.23 ps at 10 K and room temperature, respectively.This is calculated with the relation τ = mμ/e, where e is theelementary charge and m is the effective mass of carriers insingle-layer graphene. The latter is given for single-layergraphene by m = ℏkF/vF, where vF is the Fermi velocity, ℏ isthe reduced Plank constant, and kF the Fermi wave vector (=k nF , with n as the carrier density).We carried out zero-bias (i.e., zero source-drain potential,VDS) photocurrent measurements at different THz frequencies(Figure 1b). First, we studied the photoresponse of the detectorat 10 K for an incoming THz frequency of 0.3 THz. The currentresponsivity of the device, RI = IPC/P, is shown in Figure 1c, withP being the incoming power of the THz radiation and IPC themeasured photocurrent at the drain contact. It is worth notingthat, at this radiation frequency, the quality factor value Q isbelow ∼0.5, and thus the photodetector operates in theoverdamped regime (see bottom inset of Figure 1c, blueshadowed region). The experimental photoresponse exhibits anantisymmetric shape with respect to the applied top gatepotential, which flips its sign at the charge neutrality point(CNP). Such trends, together with the appearance of themaxima and minima values of the photocurrent near the CNPand a vanishing photocurrent at large gate voltages, result fromthe ambipolar charge transport in graphene and agree withprevious published works reporting nonresonant photodetec-tion in the literature.18−22 We further highlight that the lineshape of the measured current response w.r.t. the gate potentialfollows closely the trend predicted by theory,21 ΔI = −dσ/dVTG(see upper inset Figure 1c), with ΔI being the expectedphotocurrent and σ the DC channel conductivity. Thequalitative agreement between both experimental and theoreti-cal curves, with the only sign reversal occurring at the CNP,indicates that the rectified photocurrent in the device ispredominantly generated via the so-called plasmonic Dyako-Figure 2. Frequency dependence of resonant THz photodetection. (a) Normalized current responsivity, RIN, as a function of the top gate voltage at theelectron side for different frequencies in the range 2.5 THz−4.7 THz (Q ≫ 1 in the device for all these frequencies). All measurements in these fivepanels were performed at 10 K. In this panel, the measured current responsivity is normalized (RIN) with respect to the photocurrent maximumrecorded close to the CNP for an easier comparison of all recorded data at the different frequencies. (b) Resonant mode number,N, of the local minimain the RIN curves and (c) corresponding plasmon wavelength, λp, as a function of VTG−1/4 for three selected frequencies from panel (a). Solid linescorrespond to the calculated theoretical dependence following eq 4, and symbols represents the extracted values from experiments.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.3c04300Nano Lett. 2024, 24, 935−942937https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c04300/suppl_file/nl3c04300_si_003.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c04300/suppl_file/nl3c04300_si_003.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c04300/suppl_file/nl3c04300_si_003.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c04300/suppl_file/nl3c04300_si_003.pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c04300?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c04300?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c04300?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c04300?fig=fig2&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.3c04300?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asnov−Shur (DS) mechanism.17,20 Minimal discrepancies fromthe DS theory appear at large negative top gate voltages, wherethe experimental current responsivity shows a rather smallresponsivity offset ∼0.1 μA/W (an order of magnitude lowerthan the maximum RI measured), instead of the zero-photocurrent value expected from a pure plasmonic DSmechanism at large gate bias conditions. Such behavior mayresult from an additional rectification effect occurring due to thepresence of pn junctions at the metal−graphene contact.20,21Next, we measured the photoresponse of the device at 10 Kbut at a higher frequency, 4.7 THz (Figure 1d). The qualityfactor at this radiation frequency is characterized byQ≫ 1 (Q≈8.6), and thus the device operates in the resonant (weaklydamped) regime (see Figure 1c, bottom inset). Intriguingly, thecurrent photoresponse recorded at this higher frequencyexhibits not only the characteristic antisymmetric line shapewith respect to the applied gate voltage (similar to thebroadband case depicted in Figure 1c), but also markedoscillations on both electron and hole sides emerged (seearrows in the top-right and bottom-left insets in Figure 1d,respectively). Such oscillations, which are dependent on thecarrier density, constitute the hallmark of resonant operation in aFET photodetector.17,24−28 They are the result of plasmonresonances occurring in the graphene channel because of thereflection of the plasma waves at the end of the channel and theinterference of both reflected and incoming waves. Under suchconditions, the graphene device acts like a Fabry−Perotresonant cavity for propagating graphene plasmons underexternal THz excitation. The multiple ridges presented in RIare the result of the crossover from destructive to constructiveinterferences of the incoming and reflected waves. Subsequently,peaks represent waves with a number of oscillation modes thatare by one higher or lower than the neighboring crests. Themode number is tunable with both the length of the top gate(LTG) and the density of the charge carriers (controlled via theapplied gate voltage, VTG) in the system.28 Importantly, theintensity of such resonances strongly depends on the plasmoniccavity length (LTG) and the plasmon propagation length (LP,which is larger than the 1/e-decay length Ld = sτ of the plasmawave and depends on the signal-to-noise ratio at which smallmodulations of RI can still be detected), leading to two differentscenarios.17 When LTG ≲ LP, propagating plasmons can reach theend of the channel before a total decay, creating interferencesbetween the incoming and reflected waves at least at the end ofthe channel, if not along its total length (see Supporting Movie1). Such a case gives rise to different characteristic resonantmodes as a function of the carrier density or the incomingfrequency. Conversely, if LTG ≫ LP, propagating plasmons in thesystem decay before reaching the end of the cavity (seeSupporting Movie 2), giving rise to a rectified photocurrentindistinguishable to the one expected in the nonresonantscenario.17For completeness, we additionally measured the photo-response at different frequencies (range 2.5 THz−4.7 THz), allwithin the resonant regime or weakly damped scenario (Q≫ 1).Figure 2a highlights the evolution of the photoresponseoscillations within this frequency range. For simplicity andclarity, we present the normalized current responsivity, RIN, withrespect to the photocurrent maximum observed close to theCNP. Interestingly, the current photoresponse as a function ofthe gate voltage exhibits oscillations at all of these measuredfrequencies, but the visible number of oscillations stronglydepends on the THz frequency. In particular, the number ofpeaks decreases when lowering the excitation frequency. Furtherfrequency-dependent measurements can be found in SupportingInformation Note 3.The observed oscillations of the current photoresponse whensweeping VTG were further analyzed in the following way.Excited plasmons in gated two-dimensional systems follow thelinear dispersion law, ω = sk, where s is the plasma wave velocity,and k is the real part of the angular wavenumber.24,25 The plasmawave velocity is defined as= | |s emVTG (1)And resonances should emerge when the real part of thewavenumber is given by= + =kLN N2(2 1), 0, 1, 2 ...TG (2)Importantly, the effective mass, m, in single-layer gra-phene18,31 is dependent on the applied gate voltage as= = | |m kv vC VeFF Fox TG (in the former expression, Cox is thethin-oxide gate capacitance per unit area, and vF is the Fermivelocity of the charge carriers). Thus, by replacing m into eq 1,the plasma wave velocity in monolayer graphene can berewritten as=| |ikjjjjjy{zzzzzsev e VC1 4F TGox/(3)Then, using the plasmon dispersion law with eqs 2 and 3, onecan easily deduce the relation between the resonant modenumber, N, and the applied gate voltage, VTG, for single-layergraphene:= = | || |( )NLV1212ev e VCTG1/4 TG1/4F TGox (4)Following eq 4, N is expected to have a linear dependencewith ω and VTG−1/4. We verified that the experimental resonantpeaks appearing at all measured frequencies in our device(Figure 2a) follow the predicted VTG−1/4 dependence. Inparticular, Figure 2b shows the extraordinary agreementbetween the calculated theoretical dependence N(ω,VTG)given by eq 4 and the values of N extracted from theexperimental data. We note that, in comparison with systemswith parabolic bands,24−26 graphene’s linear energy-momentumresults in a notably distinct dependence of N with the appliedvoltage (systems with parabolic energy bands exhibit a relationdependence of N ∝ VTG−1/2 instead28). In consequence, the firstresonant modes in our single-layer graphene THz detector (N <6) are not accessible in the recorded VTG range at the highestmeasured frequency 4.7 THz due to the VTG−1/4 dependence of Nintroduced in eq 4. For instance, at 4.7 THz, the resonant modeN = 2 is expected to occur for gate potentials larger than 250 V,values that are not reachable in common experimental devices.Resonant modes below 6 are experimentally accessible in ourdevice only when decreasing the excitation frequency down to2.5 THz (see Figure 2b).The observed resonant modes in the THz photoresponse canbe further utilized to extract significant information on thepropagating graphene plasmons.23,28,32 Such informationincludes the plasmon lifetime (τp) and plasmon wavelength(λp). We calculated the plasmon lifetime by using the width ofNano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.3c04300Nano Lett. 2024, 24, 935−942938pubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.3c04300?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asthe characteristic resonant peaks at the half-height and the gatevoltage at which plasmon resonances arise (see SupportingInformation Note 4). The resulting values for τp were found tobe around 0.6 ps, which are larger than the aforementionedscattering time values extracted from the transport analysis.Similarly, the plasmon wavelength can be determined from themeasured resonances observed in the photocurrent with respectto the gate voltage and excitation frequency, following therelation23,28,32 ω = 2πs/λp. The obtained values for λp rangebetween 600 nm and 2.1 μm (see Figure 2c) for the studiedrange of THz frequencies (2.5−4.7 THz). These plasmonwavelength values lead to compression ratios (λo/λp, with λo = c/f being the wavelength of the incoming THz radiation in free-space) as high as 110 (see details in Supplementary Note 5). Theratio agrees well with the extreme light compression andnanoscale confinement occurring in graphene devices at THzfrequencies reported in previous works.28,32,33Finally, we measured the evolution of the plasmonicresonances at 4.7 THz when raising the temperature, T. Figure3a shows the zero-bias photoresponse as a function of the topgate potential for the hole-side (negative VTG) and the electron-side (positive VTG) at four selected temperatures within therange 10 K−300 K. Importantly, the observed resonant peaksand dips persist up to 300 K both for electron and holeFigure 3. Temperature evolution of the plasmonic resonances. (a) Zero-bias normalized photocurrent as a function of the top gate voltage at fourselected temperatures from 10 K up to 300 K (room temperature) at both the hole and electron regions for an incident radiation of 4.7 THz. For aneasier visualization, the temperature-dependent photoresponses shown in the panel are normalized with respect to the maximum near the CNP (asdone in Figure 2a), and the curves are vertically shifted. The bottom inset shows the temperature evolution of the quality factor Q = ωτ. (b) Colormapping of the normalized responsivity, RIN (after subtraction of the nonresonant back-ground), as a function of the top gate voltage at 4.7 THz for allmeasured temperatures. Vertical dashed lines highlight the evolution of the different observed resonant modes with temperature.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.3c04300Nano Lett. 2024, 24, 935−942939https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c04300/suppl_file/nl3c04300_si_003.pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c04300?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c04300?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c04300?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c04300?fig=fig3&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.3c04300?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asconduction (see also additional data in Supplementary Note 6).Thus, these measurements demonstrate the resonant detectionof THz radiation at room temperature in nonbiased FET devicesmade from single-layer graphene.The presence of the photoresponse oscillations with respectto the VTG and their detailed evolution with temperature areclearly visible in Figure 3b. In particular, this panel presents themeasured photoresponse when excluding (i.e., subtracting) thebroadband contribution for all measured temperatures withinthe range of 10 K−300 K. We stress the fact that resonant peaksand dips appear in the map approximately at the same carrierdensity (i.e., same value of VTG) for all temperatures. Thisobservation agrees well with eq 4, which does not contain anyexplicit dependence of the position of the resonances on T. Wenotice that gate-tunable photoresponse resonances shown inFigure 3a are more evident at the hole side (negative gatevoltages) than at the electron side (positive gate voltages). Thisis also seen in Figure 3b when the resonances are displayed as afunction of the temperature. We argue that this could be causedby slightly larger mobilities on the hole side with respect to theelectron side in our devices (see Supporting Information Note2). Moreover, the amplitude of the photocurrent oscillationsmeasured at room temperature in electron or hole conductiondepends ultimately on the device (Supporting Information Note7 showing stronger and more evident resonances measured atroom temperature in a second photodetector).The fact that devices made from high-quality, single-layergraphene exhibit clear and unambiguous evidence of resonantresponsivity at room temperature (including the appearance ofoscillations of the zero-bias photoresponse with respect to thegate voltage or equivalently carrier density) is extremely relevantfor applications. To date, robust signatures of this resonantregime had been reported only to occur at cryogenictemperatures in other semimetals such as bilayer graphene28or 2D electron gases made of III−V semiconductors.24−26 Onlysome experimental indications have been interpreted as arisingfrom resonant detection in III−V field-effect transistorsoperating at room temperature,34,35 but these are vague andrely on the application of a large source-to-drain bias (theapplication of a source-to-drain dc voltage or current shifts thesystem toward a resonant regime36 but also increases the noiseof the rectified signal). In contrast, our study (Figure 3) showsstrong and univocal plasmonic resonant oscillations in zero-biased photocurrent measurements performed at room temper-ature.We argue that the robust observation of the resonant regimein high-quality single-crystal graphene results from the largeroom-temperature mobility of the charge carriers in thismaterial,29,30 which for our devices is larger than 60000 cm2V−1 s−1 (see Supplementary Note 2). Such a value leads to atransport scattering time τ = 0.2 ps even at room temperatureand to a quality factor Q≫ 1 (Q > 6) at an excitation frequencyof 4.7 THz (bottom inset of Figure 3a shows the evolution of Qwith temperature in the device). Since the condition Q ≫ 1 isfulfilled, micrometer-size devices made of high-quality mono-layer graphene can robustly operate in the weakly dampedregime at room temperature and show resonant detection. Incontrast, other semiconductor materials have intrinsic carriermobilities which are around or even below 5000 cm2 V−1 s−1 atroom temperature,37 which impedes the observation of resonantdetection at room temperature. This is even the case of bilayergraphene,28 a system which also has lower intrinsic room-temperature mobility values (∼15000 cm2 V−1 s−1) thanmonolayer graphene due to the presence of additional intrinsicscattering sources including shear phonon scattering38 orsignificantly larger electron−hole collisions.39In summary, we have studied the zero-bias photoresponse ofhigh-mobility monolayer graphene FETs subjected to THzradiation. The operation of the devices is perfectly tunedbetween nonresonant and resonant regime depending on thefrequency of the incoming radiation. In particular, the resonantregime is univocally demonstrated by the measured oscillationspresent in the gate-voltage-dependent photocurrent. Theseoscillations are dependent on both the carrier density in thechannel and the frequency of the THz radiation. Wedemonstrate that such univocal fingerprints of resonant THzphotodetection are visible not only at cryogenic temperaturesbut also at room temperature. To the best of our knowledge, thisis the first time that resonant THz photodetection has beenrobustly observed at room temperature without the applicationof a large drain current bias (which is undesirable for a properdetector operation).From an application point of view, these findings pave the wayfor the design and development of a new generation of(graphene-based) plasmonic resonance photodetectors operat-ing at room temperature. The application space of such systemsis significant in the THz and mid-infrared regime6,7 allowing therealization of emerging and potential technologies at theserelatively unexploited but relevant frequencies, includingmodulators, filters, polarizers, emitters, and selective photo-detectors, among many others, as well as the confinement andmanipulation of the electromagnetic fields below the classicaldiffraction limit.32■ ASSOCIATED CONTENT*sı Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c04300.Additional fabrication details, transport data, calculations,and experimental results (PDF)Two supporting movies that show plasma-waves prop-agating in plasmonic cavities (MP4-1, MP4-2)■ AUTHOR INFORMATIONCorresponding AuthorJuan A. Delgado-Notario − Department of Applied Physics,University of Salamanca, Salamanca 37008, Spain;orcid.org/0000-0001-9714-8180; Email: juanandn@usal.esAuthorsJosé M. Caridad−Department of Applied Physics, University ofSalamanca, Salamanca 37008, Spain; Unidad de Excelenciaen Luz y Materia Estructurada (LUMES), Universidad deSalamanca, Salamanca 37008, Spain; orcid.org/0000-0001-8943-1170Óscar Castelló − Department of Applied Physics, University ofSalamanca, Salamanca 37008, Spain; Unidad de Excelenciaen Luz y Materia Estructurada (LUMES), Universidad deSalamanca, Salamanca 37008, Spain; orcid.org/0009-0009-8889-6270Sofía M. López Baptista − Department of Applied Physics,University of Salamanca, Salamanca 37008, SpainTakashi Taniguchi − Research Center for MaterialsNanoarchitectonics, National Institute for Materials Science,Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.3c04300Nano Lett. 2024, 24, 935−942940https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c04300/suppl_file/nl3c04300_si_003.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c04300/suppl_file/nl3c04300_si_003.pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c04300?goto=supporting-infohttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c04300/suppl_file/nl3c04300_si_003.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c04300/suppl_file/nl3c04300_si_001.mp4https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c04300/suppl_file/nl3c04300_si_002.mp4https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Juan+A.+Delgado-Notario"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0001-9714-8180https://orcid.org/0000-0001-9714-8180mailto:juanandn@usal.esmailto:juanandn@usal.eshttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Jose%CC%81+M.+Caridad"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0001-8943-1170https://orcid.org/0000-0001-8943-1170https://pubs.acs.org/action/doSearch?field1=Contrib&text1="O%CC%81scar+Castello%CC%81"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0009-0009-8889-6270https://orcid.org/0009-0009-8889-6270https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Sofi%CC%81a+M.+Lo%CC%81pez+Baptista"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takashi+Taniguchi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.3c04300?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asTsukuba 305-0044, Japan; orcid.org/0000-0002-1467-3105Kenji Watanabe − Research Center for Electronic and OpticalMaterials, National Institute for Materials Science, Tsukuba305-0044, Japan; orcid.org/0000-0003-3701-8119Hartmut G. Roskos− Physikalisches Institut, Johann WolfgangGoethe-Universität, Frankfurt am Main D-60438, Germany;orcid.org/0000-0003-3980-0964Complete contact information is available at:https://pubs.acs.org/10.1021/acs.nanolett.3c04300NotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSAuthors thank the support from the Ministry of Science andInnovation (MCIN) and the Spanish State Research Agency(AEI) under grants (PID2021-126483OB-I00, PID2021-128154NA-I00) funded by MCIN/AEI/10.13039/501100011033 and by “ERDF A way of making Europe”. Thiswork has been also supported by Junta de Castilla y Leóncofunded by FEDER under the Research Grant numbersSA103P23. J.M.C. acknowledges financial support by the MCINand AEI “Ramón y Cajal” program (RYC2019-028443-I)funded by MCIN/AEI/10.13039/501100011033 and by “ESFInvesting in Your Future”. J.M.C. also acknowledges financial ofthe European Research Council (ERC) under Starting grant ID101039754, CHIROTRONICS, funded by the EuropeanUnion. Views and opinions expressed are however those ofthe author(s) only and do not necessarily reflect those of theEuropean Union or the European Research Council. Neither theEuropean Union nor the granting authority can be heldresponsible for them. K.W. and T.T. acknowledge supportfrom the JSPS KAKENHI (Grant Numbers 21H05233 and23H02052) and the World Premier International ResearchCenter Initiative (WPI), MEXT, Japan. The work in Frankfurt issupported by DFG projects RO 770/40-2 and RO 770/53-1.J.A.D.-N. thanks the support from the Universidad de Salamancafor the María Zambrano postdoctoral grant funded by the NextGeneration EU Funding for the Requalification of the SpanishUniversity System 2021−23, Spanish Ministry of Universities.Authors also acknowledge USAL-NANOLAB for the use ofClean Room facilities.■ REFERENCES(1) Buron, J. 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