Letter pubs.acs.org/NanoLett
Tuning Chemical Potential Difference across Alternately Doped Graphene p−n Junctions for High-Efficiency Photodetection Li Lin,† Xiang Xu,§ Jianbo Yin,† Jingyu Sun,† Zhenjun Tan,†,⊥ Ai Leen Koh,∥ Huan Wang,† Hailin Peng,*,† Yulin Chen,*,§,‡ and Zhongfan Liu*,† †
Center for Nanochemistry, Beijing Science and Engineering Center for Nanocarbons, Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China ‡ Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford OX1 3PU, U.K. ∥ Stanford Nano Shared Facilities, Stanford University, Stanford, California 94305, United States § State Key Laboratory of Low Dimensional Quantum Physics, Collaborative Innovation Center of Quantum Matter and Department of Physics, Tsinghua University, Beijing 100084, China ⊥ Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, China S Supporting Information *
ABSTRACT: Being atomically thin, graphene-based p−n junctions hold great promise for applications in ultrasmall highefficiency photodetectors. It is well-known that the efficiency of such photodetectors can be improved by optimizing the chemical potential difference of the graphene p−n junction. However, to date, such tuning has been limited to a few hundred millielectronvolts. To improve this critical parameter, here we report that using a temperature-controlled chemical vapor deposition process, we successfully achieved modulation-doped growth of an alternately nitrogen- and boron-doped graphene p− n junction with a tunable chemical potential difference up to 1 eV. Furthermore, such p−n junction structure can be prepared on a large scale with stable, uniform, and substitutional doping and exhibits a single-crystalline nature. This work provides a feasible method for synthesizing low-cost, large-scale, high efficiency graphene p−n junctions, thus facilitating their applications in optoelectronic and energy conversion devices. KEYWORDS: Large and controllable chemical potential difference, graphene p−n junction, high-efficiency photodetection, alternately nitrogen- and boron-doped graphene
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ants,9,12−14 but such approaches have severe limitations in either introducing enough charge carriers or mass production, thus limiting their practical applications. Alternatively, we recently developed a modulation-doped chemical vapor deposition (CVD) method and successfully realized the synthesis of single-crystalline graphene i−n junctions,11 which provides a new route for realizing more effective graphene p−n junctions for photodetectors. Nevertheless, the performance of such i−n junction photodetectors still show a relatively low photoresponsivity (several tens of mA W−1) due to the small potential difference across the junction that contributes to
raphene has emerged as a promising candidate for highperformance electronic and optoelectronic devices due to its phenomenal properties1−4 originating from the unique twodimensional massless Dirac band structure. In particular, the ultrahigh carrier mobility and broadband photon absorption make it exceptionally attractive for photodetectors with excellent characteristics, such as reaction time and quantum efficiency.1,5−7 For the graphene p−n junctions used in a photodetector, the need to separate the photogenerated electron−hole pairs is critical, in order to inhibit their fast recombination because their lifetime is typically quite short.8 However, to date, making graphene p−n junctions with excellent characteristics still remains a great challenge.9−12 For instance, graphene p−n junctions could be fabricated by introducing an external gate or adsorbed chemical dop© XXXX American Chemical Society
Received: February 24, 2016 Revised: June 15, 2016
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DOI: 10.1021/acs.nanolett.6b00803 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 1. (a) (top) Schematic drawing of the modulation-doped CVD route to achieve the growth of alternately nitrogen- and boron-doped graphene films, where the nucleation and sequential epitaxial growth steps are shown, respectively. (bottom) Schematic illustration of temperaturedependent CVD growth of alternately BNG p−n junction to tune the chemical potential difference across as-formed p−n junctions. Notably, hightemperature growth usually resulted in a low doping level in graphene. (b) Representative AES point-analysis results of NG islands and BG intervals in BNG structures. The inset shows representative SEM image of the alternately BNG film on copper foils for AES measurement. Scale bar, 5 μm. (c) AES B 1s and N 1s line-analysis result of the BNG p−n junction along the white dashed line marked in the inset of panel b, suggesting isolated feature of the elemental distribution. (d) The typical SEM image of alternately BNG p−n junction transferred onto 300 nm SiO2/Si substrate. Scale bar, 10 μm. NG and BG portions can be identified by the contrast. Th inset shows typical OM image of alternately BNG film. Scale bar, 10 μm. The white arrow denotes the presence of SiO2 substrate. (e) Large-area alternately BNG film transferred onto a 4-in. silicon wafer. (f) Typical Raman spectra of NG (blue) and BG (red) portions of alternately BNG p−n junction grown at 1000, 950, and 900 °C respectively, collected with 514 nm incident laser. The Raman spectra from the interior to the exterior (along the direction of arrow) were acquired at the NG portions of 900 PN, 950 PN, and 1000 PN and at BG portions of 1000 PN, 950 PN, and 900 PN, respectively. (g) The 2D band-position mapping of Raman spectroscopy over a region of the alternately BNG p−n junction (900 PN). Scale bar, 5 μm. The inset shows the corresponding SEM image of alternately BNG p−n junction transferred onto 300 nm SiO2/Si for Raman mapping.
extraction and separation of photogenerating carriers, as well as the low intrinsic absorption in monolayer graphene.11,15−17 Theoretically, there has been no consensus by far on the exact physical mechanisms responsible for light-to-current conversion processes for all graphene p−n junctions. The prevailing photovoltaic or photothermoelectric (PTE) mechanisms are responsible for different types of graphene-based photodetectors and device test conditions.7,9,10,18 According to the PTE effect,9,18,19 optimization of the conversion efficiency is possible by tuning the chemical potential difference across the graphene p−n junction, which alters the thermoelectric power of each component, thereby tailoring the temperature gradient of hot electrons and corresponding photovoltage. However, the degree of the tunability over the chemical potential difference by electric gating is very limited,9 and the work function difference of the CVD-grown graphene i−n junction (where one portion was intrinsic graphene) can also be improved if we make true p−n junctions.11 Doping graphene with heteroatoms via CVD is an effective way to tune the doping levels in graphene in a simple and stable manner.20−24 By insertion of boron or nitrogen atoms into the carbon lattices, p-type or n-type doped graphene can be selectively obtained. The doping level can be simultaneously controlled by tuning the amount of dopants.25 In the present study, we develop a modulation-doped CVD route to achieve the growth of continuous alternately nitrogenand boron-doped graphene (BNG) films on copper foils. The chemical potential difference across the as-formed alternately BNG p−n junction can be tuned to as high as 1 eV by tailoring the growth temperature to gain fine control over the doping level of each part of the junction, as confirmed by angle-
resolved photoemission spectroscopy (ARPES) measurements. Systematic investigation on the photogeneration at the p−n junction demonstrates that the PTE mechanism still plays a dominant role in the photovoltage generation process and that the chemical potential difference across the p−n junction can be tuned to realize an enhanced PTE conversion. These results may open up the possibility of using as-fabricated graphene p− n junction in next-generation photodetectors and optoelectronic and energy conversion devices. Figure 1a illustrates the modulation-doped CVD strategy for the growth of alternately BNG films, enabling a graphene p−n junction with tunable chemical potential difference.11,26 Note that the modulation-doped method is for the integration of intricate n- and p-type segments in a tuned manner, which has already been reported in achieving nanostructure junctions with single-crystalline nature.11,26 This process relies on the unique features of 2,4,6-tri(2-pyridyl)-1,3,5-triazine (TPTZ) and phenylboronic acid, which can form nitrogen-doped graphene (NG) and boron-doped graphene (BG), respectively, on polycrystalline copper foils at elevated growth temperatures. To begin with, discrete NG islands are grown on annealed copper foils by introducing TPTZ in a controllable fashion, serving as seeds for the subsequent epitaxial growth of BG (Figure S1). Notably, the nucleation density of NG nuclei could be readily adjusted (from 0.4 to 10−3 μm−2) by tuning the H2 gas flow rate (Figure S2). In the following step, the laterally grafted growth of BG from the existing NG islands occurs by the subsequent introduction of phenylboronic acid as a precursor, giving rise to a continuous alternately BNG film upon the coalescence of as-deposited grains. To suppress the spontaneous nucleation of BG at this stage, the interval space B
DOI: 10.1021/acs.nanolett.6b00803 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 2. (a) SEM image of an alternately BNG p−n junction for ARPES measurements. Scale bar, 5 μm. (b) Stacked plots of ARPES spectral intensity mapping as a series of binding energy, showing an obvious contrast of n- and p-type domains. (c,d) 3D intensity plots of the photoemission spectra of n- and p-type domains. The Dirac point is well below the Fermi level surface in the n-type portion, and the Dirac point is above the Fermienergy in the p-type portion. (e, f) Band dispersions measured at the Dirac points of n- and p-type. Yellow dashed lines indicate that in the NG portion, the Dirac point is 650 meV lower than the Fermi level (shown in panel e), while in the PG portion, the Dirac point is 350 meV higher than the Fermi level (shown in panel f).
calculated to be ∼3% from the AES peak intensities. Optical microscopy (OM) image (Figure 1d, inset) of a transferred alternately BNG sample on Si/SiO2 substrate verifies the synthesis of a continuous monolayer film with no optical contrast observed (compared with the SEM image taken on copper, as shown in Figure 1d). Such results demonstrate the high yield of the BNG p−n structure obtained from this modulation-doping CVD method. Moreover, upon introducing methane to initiate the growth of intrinsic graphene, complex in-plane heterostructures consisting of spatially well-defined intrinsic, NG, and BG portions with alternating contrast in homocentric growth rings (i−n−p junctions) can be achieved through multiple modulation cycles (Figure S4). Notably, the fabrication of a 4-in. wafer-scale BNG film indicates the possibility of scale-up production of p−n junction arrays by using a roll-to-roll method and seed-assisted template method (Figure 1e), which is important for future large-scale integration and applications (Figure S5).11,30,31 We use Raman spectroscopic characterization to probe the temperature dependence of dopant concentrations and the feasibility in tuning the chemical potential difference of asobtained alternately BNG samples.32 Single-point Raman spectra acquired at the NG and BG portions of 1000 PN, 950 PN, and 900 PN are presented in Figure 1f, all of which exhibit a prominent D band (∼1351 cm −1 ) and an accompanying D′ band (∼1620 cm−1) stemming from the elastically scattered photoexcited electron generated by foreign atoms in the graphene matrix and intravalley double resonance scattering processes, respectively.33 The D band is informative with respect to the level of dopant or defects in the graphene lattice, and the intensity ratio of the D band to G band (ID/IG ratio) could reflect the dopant concentration in graphene.33,34 It can be seen from the Raman spectra that the ID/IG ratios in
between NG islands could be predefined by tuning the growth time of NG at the first step (Figure S3). More significantly, to achieve fine-tunability over the chemical potential difference, temperature-dependent CVD growth of BNG p−n junction was carried out, since previous reports indicated that a hightemperature growth usually resulted in a low doping level in graphene due to the difference in the bonding energies between the C−C bond and C−N and C−B bonds.27,28 In this regard, sample 1000 PN, 950 PN, and 900 PN denotes the alternately BNG p−n junction fabricated at 1000, 950, and 900 °C, respectively. The inset of Figure 1b presents a typical scanning electron microscope (SEM) image of 900 PN grown on copper foil, showing individual NG and BG regions and a high yield of alternately BNG p−n junctions. Due to the monolayer nature of the as-grown graphene p−n junction, the contrast is determined by the work function of each portion. The NG regions can be easily distinguished from the BG regions by image contrast, with NG islands showing a bright contrast and the BG intervals as much darker color. In addition, the asformed NG grain exhibited a dendritic structure, indicating the dominance of diffusion-limited growth kinetics in graphene growth, caused by the supply of carbon source and flow rate of hydrogen.29 Auger electron spectroscopy (AES) was employed to obtain the spatial distribution of the specific elements within the as-grown alternately BNG films. Figure 1b shows representative AES point-analysis results of NG islands and BG intervals in BNG structures, where the B 1s peak centers at ∼175 eV and the N 1s peak locates at 382 eV. Apparently, NG islands (the bright regions in inset of Figure 1b) display no boron signals, and the BG regions exhibit no nitrogen signals, which is consistent with further AES line-analysis results (Figure 1c). The atomic concentration of heteroatoms is C
DOI: 10.1021/acs.nanolett.6b00803 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 3. (a) Low-magnification TEM image of a discrete BNG grain. The grain is rendered orange to enhance the contrast. The circles mark the positions where SAED patterns in panels b−e were collected. Observed pores belong to the supporting membrane rather than graphene sample. SAED patterns were captured with ∼200 nm aperture. Scale bar, 2 μm. (b−e) SAED patterns of specified spots in panel a. The patterns shown in panels b and e were collected from the points marked by the red circles, corresponding to the BG portion, while the patterns in panels c and d are related to the NG portion. The inset shows the intensity profile of the diffraction pattern along the red dashed line, which confirms the monolayer nature of as-fabricated p−n junction. Scale bar, 5 nm−1. (f) Histogram of angle distribution from extensive SAED patterns within the entire grain. (g, h) Aberration-corrected and monochromated TEM image of the BG (g) and NG (h) portions of one alternately BNG p−n junction, showing a perfect carbon lattice with 6-fold symmetry. All the TEM characterizations were performed on sample of 900 PN. Scale bar, 2 nm.
the NG portions decrease from 0.49 (900 PN) to 0.16 (1000 PN), suggesting the reduction of the nitrogen concentration upon increasing the growth temperature. This finding is in good agreement with the results from X-ray photoelectron spectroscopy (XPS) (Figure S5). Similar phenomenon was also found in the Raman spectra of the BG portions. The observation of strong 2D band is indicative of the high quality of the as-grown doped graphene.33 The noticeable 2D band shift in fact reveals the shift of the Fermi level of BG and NG portions,32 substantiating fine-tunability of the chemical potential difference in the alternately BNG p−n junction via simply tailoring the growth temperature. Raman mapping of 2D band position demonstrates the uniformity of the doping (Figure 1g), where the 2D band of NG portions exhibits a remarkable downshift compared with that for BG portions. The substitutional doping of heteroatoms in the graphene skeleton is further confirmed by our XPS (Figure S6 and Figure S7) and X-ray absorption near edge structure (XANES) data (Figure S8). A direct method to examine the doping level of graphene is to probe its electronic band structure (e.g., at the Dirac cone dispersion around K points in the 2D Brillouin zone). Although conventional ARPES is a powerful tool to directly visualize the electronic structure of solids, its spatial resolution (typically hundreds of micrometers) makes it unsuitable for this study because the p- and n-types of graphene domains are only a few micrometers in lateral size. Instead, we made use of the state-ofthe-art spatially resolved ARPES technique, which is a recently developed ARPES method with sub-micrometer spatial resolution (achieved by focusing the incident photon beam to a small spot on the sample) that suits our need. With the sub-micrometer resolution, we were able to see the distribution of p- and n-domains of an alternately BNG film from an ARPES spectral intensity mapping, which reflects the local 2D density of state at each pixel (Figure S9). Although it is intuitive to identify domains with different dopants through
discrimination of the 3D density of state near the Fermi level featured by the linear dispersions of graphene, a more convenient way is to focus on the state from the copper substrate (several electronvolts below the Fermi level) due to the varied transmissivity of different domains. In Figure 2b, the NG portion and the BG portion showed an obvious contrast at a series of binding energies down to several electronvolts below the Fermi level, which resembles the representative SEM image of alternately BNG film for measurement (Figure 2a). After mapping out the spatial distribution, we were able to choose the p- and n-type graphene domains from the ARPES morphologic image and performed local ARPES measurements, which are shown in Figure 2c,d,e,f. Evidently, in the NG portion (Figure 2c,e), the Dirac point is well below the Fermi surface (by ∼650 meV) and shows an n-type electronic structure; while in BG portion (Figure 2d,f), the Dirac point is above the Fermi-energy (by ∼350 meV), showing clear p-type doping. Remarkably, the difference of the doping levels between the p- and n-type portions in our grown p−n junction is as high as 1 eV, which is much larger than the gating method used before,35 the p−i junctions in previous works, or other methods (see Table S1).11 It is well-known that grain boundaries in polycrystalline graphene strongly impede electronic transport and scatter the photogenerated carriers.36 We employ transmission electron microscopy (TEM) imaging and selected area electron diffraction (SAED) to characterize the quality and crystalline nature of the as-grown alternately BNG p−n junction. In this regard, discrete core−shell BNG grains were grown in a controllable manner to allow us to easily locate the position of the junction in a TEM and transferred on TEM grids with identifier markers. The sample was first studied using the SEM to locate grains of interest and positions of the junctions for subsequent TEM analyses. The representative SEM image of one such grain (Figure S10a) shows that it consists of an NG core and a BG shell with a typical width of ∼1 μm. Having D
DOI: 10.1021/acs.nanolett.6b00803 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 4. (a) Schematic view of transistor of graphene p−n junction for transport and photovoltage measurements. Red arrows indicate the shifting direction of hot carriers under the PTE scheme. A focused 532 nm laser (∼1 μm, ∼250 μW) was used to illuminate the junction area to excite photogenerated carriers. (b) Typical transfer characteristic curve of the 950 PN. The inset shows SEM image of measured device, with NG and BG potions false colored red and blue, respectively. The scale bar is 5 μm. (c) The corresponding photovoltage mapping image of the device in panel b. The inset shows the corresponding 3-D view of the scanning photovoltage image of the same device. Such result is mapped by presenting the value of photovoltage across the white dashed line in panel c from front view. (d) Calculated intensity of photovoltage as a function of chemical potentials μ1 and μ2 of the p−n junction based on the PTE effect. The upper curves denote the plots of Seebeck coefficients of NG (blue) and BG (yellow) as functions of chemical potential of the p−n junction. (e) Plot of photovoltage as a function of gate voltage (Vg), exhibiting a single peak with two polarity reversals, suggesting that a PTE effect, rather than PV effect, dominates the photoconversion process at the graphene p−n junction. The measurements were performed on the same device in panels b and c. The inset shows theoretical calculation of photovoltage versus Vg under the PTE scheme. (f) The statistics of experimental responsivities in three types of devices, where the 950 PN with a moderate chemical potential difference exhibited a stronger responsivity. The inset shows the calculation result of the responsivity of the p−n junction as a function of doping level of the p−n junction under the PTE scheme, which shows that the responsivity is optimized when the chemical potential is doped by ±200 meV relative to the Dirac point.
the graphene lattice is preserved. These observations are consistent with the XPS and XANES results,37 which suggest substitutional doping as the main dopant mode. The dopant concentration, which is estimated to be 3% from AES data, is below the detection limit of TEM-based spectroscopy techniques. The high quality of the as-made alternately BNG p−n junction is expected to facilitate efficient photoconversion at the junction under illumination. Figure 4a presents a schematic illustration of a graphene p−n junction photodetector, where one single-crystalline p−n junction (1000 PN, 950 PN, 900 PN) was patterned and embedded into a two-terminal device (Figure 4b, inset). A focused 532 nm laser (∼1 μm, ∼250 μW) was used to illuminate the junction area to excite photogenerated carriers (see Supporting Information). Transport measurements were first performed to evaluate the electrical properties of these three types of graphene p−n junctions as well as to probe their chemical potential differences. The transfer characteristic curves of 950 PN (Figure 4b), 1000 PN, and 900 PN devices (Figure S12) all exhibit two maxima corresponding to the charge neutrality points (Dirac points) of the NG and BG, respectively, which is the hallmark of a graphene p−n junction. The distance between the two charge neutrality points corresponds to the difference in the chemical potential of the NG and BG portions. Using the formula E F = ℏνF πn (ref 2), we manage to extract the chemical
identified the locations of the grains, we then used TEM for structural characterization. Figure 3a presents the lowmagnified TEM image of a discrete grain, which could be clearly identified from the support due to the image contrast. To identify whether this grain is a single crystal, we carried out a series of SAED measurements on eight points across the entire domain. Four representative SAED patterns obtained at the positions marked in Figure 3a are presented in Figure 3b−e. It is noted that both the patterns in Figure 3b,e were collected at the points marked by the red circles, namely, corresponding to the BG portions, while the other two patterns are related to the NG portions. These patterns clearly exhibit the same orientation of ∼24°. The histogram of orientation distributions extracted from our SAED measurements in Figure 3f reveals less than 1.5° rotation throughout entire domain. Such perfect coincidence of all the diffraction patterns suggests the singlecrystal nature of the alternately BNG p−n junction11 and the absence of grain boundaries and precipitation between the BG and NG regions. Similar measurements on the lattice distances reveal the preservation of graphene lattices after doping with heteroatoms (Figure S10b). Additionally, the atomic structure of the alternately BNG p−n junction was examined by using aberration-corrected TEM (Figure 3g,h; Figure S11), where fast Fourier transforms (FFTs) of images, taken at regions corresponding to both NG and BG portions, exhibit the same lattice orientation. Furthermore, the 6-fold symmetry of E
DOI: 10.1021/acs.nanolett.6b00803 Nano Lett. XXXX, XXX, XXX−XXX
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with the efficiency of photoconversion. Note that the unit of chemical potentials is the neutrality region magnitude (see Supporting Information for details). With respect to the experiment, μ1 and μ2 can be tuned simultaneously but not in the same pace by sweeping Vg. Consequently, the photovoltage is expected to change polarity twice as Vg changes (Figure 4e, inset), which is consistent with our experimental observation. The statistics of experimental responsivities in three types of devices are presented in Figure 4f, where the 950 PN device with a moderate chemical potential difference exhibits a remarkably stronger photoresponsivity under illumination. Additionally, based on the results in Figure 4d, the inset of Figure 4f plots the calculation results of responsivity at the p−n junction as a function of doping level (here we assume that the p-type and n-type domains share the same doping level, that is, |μ1| = |μ2|, when no gate voltage is applied on the sheet), revealing that the photoresponsivity is optimized when |μ1| = |μ2| ≈ 200 meV, which is consistent with experimental results. Clearly from experiments and by the calculation, we showed that chemically tuning the chemical potential difference of the graphene p−n junction in a tailored manner can greatly improve the photoconversion efficiency, which would facilitate applications of graphene in high-efficiency photodetection devices. In summary, we have developed a novel CVD approach to achieve the synthesis of alternately BNG p−n junction with tunable chemical potential differences. The advantage of our route lies in the combination of the modulation-doped synthetic strategy and temperature-tailored CVD process. The as-obtained alternately BNG heterostructure exhibits a singlecrystalline nature, with a considerable tuning range of the chemical potential difference across the p−n junction (as high as 1 eV). Such unique features of as-grown materials allow us to realize efficient photoconversion of corresponding devices under the PTE scheme, as confirmed by our experimental and theoretical investigations. With the practical scalability of the CVD method, our results are hence a significant step forward in realizing the ultimate promise of graphene in highperformance electronic, optoelectronic, and energy-conversion devices.
potential of each portion from these curves, revealing that the work function difference can be tuned from 270 (1000 PN) to 700 meV (900 PN). The carrier mobilities of each portion extracted from the curve near each Dirac point are summarized in Figure S10. Note that the small inconsistency in the values of chemical potential difference obtained from transport measurements and ARPES results is presumably due to the nonuniform p-type doping in NG and BG caused by the adsorbed oxygen and water during a graphene transfer procedure, which can be reduced by high-temperature treatment, as confirmed by the Raman spectroscopy (Figure S13).38 Spatially resolved photovoltage mapping of the devices (Figure 4c and Figure S14) using a home-built scanning photocurrent microscope (see Methods) was also carried out. As shown in Figure 4c, the photovoltage appears in three separate zones: source, drain, and p−n junction, where its intensity at the p−n junction area is evidently stronger than that at the graphene/electrode area, suggesting an efficient photoconversion process occurring at the p−n junction. The photovoltage generated at the graphene/electrode zones, where the photovoltaic (PV) effect is deemed to play a primary role, is relatively weak (Figure 4c, inset). This is possibly due to the small shift of the Fermi level of highly doped graphene (doping from metal).10 All the photovoltage-related measurements presented in this work were taken at zero source−drain bias. Figure 4e plots the photovoltage generated at the p−n junction area (950 PN) as a function of gate voltage. It shows a single peak with two polarity reversals, suggesting that the PTE effect dominates the photovoltage conversion process, rather than the PV effect, where the polarity of the photovoltage is changed only once during the sweeping of gate voltage.9,18 Such phenomena are also observed in 1000 PN and 900 PN devices (Figure S15), verifying that the PTE effect is dominant in the photoconversion process of all three types of p−n junctions. To gain a better understanding of the photoconversion mechanism for the graphene p−n junctions, we can analyze the photoconversion efficiency based on the PTE effect.9,13,39,40 As briefly introduced below, the low cooling efficiency through the electron−phonon coupling process in graphene causes a temperature gradient, and a p−n junction is introduced to break the symmetry of the thermal current; in addition, the sign and magnitude of the PTE voltage at p−n junctions is highly dependent on the difference in the Seebeck coefficients of each portion, which can be written as VPTE = (S2 − S1)ΔT, and the classical Mott formula gives the Seebeck coefficient of a free electron gas as S=−
π 2kBT ∂ ln R 3|e| ∂μ
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b00803. Experimental details, reported chemical potential differences of graphene p−n junctions, growth protocol and CVD system for growing alternately BNG p−n junction, nucleation density of NG islands as a function of flow rate of H2, coverage of NG as a function of growth time of NG nuclei, growth of i−n−p junction through multiple modulation cycles, illustrations of potential application of large-area grown p-n junctions in array photodetector, temperature dependence of the doping concentration and doping type of as-formed NG flims, doping concentration and doping type of BG film, XANES characterization of doping configuration of nitrogen in NG formed at different temperatures, 2D contrast between N-doped and B-doped regions, structural characterization of as-grown graphene p−n junction, TEM and HRTEM characterizations of
E = EF
where T is the sample temperature, kb is the Boltzmann constant, and EF is the Fermi level energy. In our simulation, we assume that the temperature difference is a constant and set the measurement conditions as follows: spot size of the laser l0 = 1 μm, temperature of the measurement = 300 K. The photovoltage intensity of the PTE effect (VPTE) is therefore proportional to the difference in the bilateral Seebeck coefficients, which are dependent on the chemical potentials of each region, μ1 and μ2. By mapping out the normalized VPTE versus the chemical potentials of both sides at the alternately BNG p−n junction (Figure 4d), we are able to correlate the chemical potential of the p−n junction well F
DOI: 10.1021/acs.nanolett.6b00803 Nano Lett. XXXX, XXX, XXX−XXX
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alternately BNG p−n junction, transfer curves of devices of alternately BNG p−n junction, 2D band position of the nitrogen-doped and boron-doped graphene as function of the test temperature, photovoltage mapping of devices of the alternately BNG p−n junction, and photovoltage versus applied gate voltage when the laser is located at metal contacts and p−n junction PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: zfl
[email protected]. *E-mail:
[email protected]. *E-mail:
[email protected]. Author Contributions
L.L. and X.X. contributed equally to this work. Z.F.L and H.L.P designed the experimental part of the project. L.L realized growth of alternating nitrogen- and boron-doped graphene p−n junctions and conducted characterizations of as-made graphene p−n junction. X.X. and Y.L.C. carried out ARPES measurements. L.L., J.B.Y., and Z.J.T. conducted the experimental portion of the photovoltage measurement of the graphene p−n junctions, and L.L., J.B.Y. and H.L.P. analyzed the measurement results. X.X. conducted the theoretical calculation of the photoconversion at the graphene p−n junction with Y.L.C. A.L.K. and H.L.P. conducted the TEM, HRTEM, and aberration-corrected HRTEM experiments. L.L., J.Y.S., X.X., H.L.P., Y.L.C. and Z.F.L. cowrote the manuscript. All the authors participated in the data analysis. Funding
This work was financially supported by the National Basic Research Program of China (Nos. 2012CB933404, 2013CB932603, and 2014CB932500), the National Natural Science Foundation of China (Nos. 51432002, 51520105003, 21222303, 51121091, and 51362029), National Program for Support of Top-Notch Young Professionals, and Beijing Municipal Science & Technology Commission (Nos. Z151100003315013, Z131100003213016, and Z141103004414103). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Part of this work was performed at the Stanford Nano Shared Facilities.
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