High-Performance 2D MoS2 Phototransistor for Photo Logic Gate and

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High-Performance 2D MoS2 Phototransistor for Photo Logic Gate and Image Sensor Young Tack Lee,†,# Ji-Hoon Kang,‡,§,# Kisung Kwak,‡,# Jongtae Ahn,‡,∥ Hyun Tae Choi,‡ Byeong-Kwon Ju,§ Seyed Hossein Shokouh,∥ Seongil Im,∥ Min-Chul Park,*,‡,⊥ and Do Kyung Hwang*,‡,⊥ †

Department of Electronic Engineering, Inha University, Incheon 22212, Republic of Korea Center of Optoelectronic Materials and Devices, Post-Silicon Semiconductor Institute and ⊥Division of Nano & Information Technology, KIST School, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea § Display and Nanosystem Laboratory, College of Engineering, Korea University, Seoul 02841, Republic of Korea ∥ Institute of Physics and Applied Physics, Yonsei University, Seoul 03722, Republic of Korea

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S Supporting Information *

ABSTRACT: Two-dimensional (2D) MoS2 is a representative ntype transition-metal dichalcogenide (TMD) semiconductor that has great potential for future nanoscale electronic and optoelectronic applications. Here, we report a high-performance MoS2 phototransistor that exhibits a photoresponse in the 400− 700 nm range with the maximum responsivity of over 1 × 104 A/ W. As a more sophisticated optoelectronic application than a simple unit device, it is implemented in a photoinverter (NOT logic gate) connected to an external resistor, which clearly shows photoinduced static and dynamic characteristics. Furthermore, we demonstrate a prototype visible imager using the MoS 2 photoinverter as imaging pixels as an excellent example of advanced developments in an optoelectronic system based on the 2D semiconductors. KEYWORDS: two-dimensional van der Waals materials, MoS2, graphene contact, phototransistor, photoinverter, image sensor

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technologies in our daily lives like imaging, remote sensing, optical communications, and so on. In 2012, the first monolayer and multilayer MoS2 phototransistors recorded responsivity values of 7.5 mA/W (monolayer)9 and over 100 mA/W (multilayer),10 respectively. In 2013, Lopez-Sanchez et al. demonstrated a monolayer MoS2 phototransistor with very high responsivity of 880 A/W.12 Several research groups also have reported regularly on a variety of MoS 2 -based optoelectronic devices.23−28 However, a demonstration of more advanced applications beyond basic unit devices, ones such as photo logic gate or image sensor, has been relatively rare. In this study, we fabricated a multilayer MoS2 phototransistor with graphene source/drain (S/D) electrodes. Such phototransistor exhibited a photoresponse in 400−700 nm range with the responsivity reaching over 1 × 104 A/W. As a more sophisticated optoelectronic application than a simple unit device, it was implemented in a photoinverter (NOT logic gate) connected to an external resistor, which clearly showed photoinduced static and dynamic characteristics. Furthermore, we demonstrate a prototype visible imager under green light

ince the discovery of graphene, two-dimensional (2D) layered van der Waals (vdW) materials have been extensively studied with much attention owing to their unique and interesting physical properties such as thickness-dependent energy band gap,1,2 nature of dangling−bond−free surface,3 giant magnetoresistance (GMR),4 and valley (pseudospin).5−7 A considerable amount of effort has been devoted on fundamental studies of the materials as well as development in future nanoelectronic or nanophotonic technologies. As a result, a remarkable progress has been made of late in the field of 2D vdW materials: (1) synthesis of various 2D layered vdW materials such as transition-metal dichalcogenides (TMDs) of MoS2, MoSe2, MoTe2, WSe2, WTe2, PtSe2, and PdSe2, posttransition-metal dichalcogenides (PTMDs) of SnS2 and SnSe2, and black phosphorus (BP) and (2) demonstration of prototype electronic/optoelectronic applications.6,8−20 Despite introductions of many new 2D vdW materials, MoS2 still remains one of the most promising semiconductors. MoS2based field effect transistors (FETs) have shown satisfactory carrier mobility values with high on/off current ratios at room temperature and thus can be used as a building block in electronic circuitry such as in amplifier and logic circuit.21,22 In addition, MoS2 also has a great potential for optoelectronic application, especially in photodetectors vital in various © XXXX American Chemical Society

Received: July 30, 2018 Published: December 3, 2018 A

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optical fiber as the optical probe, and a semiconductor parameter analyzer, as in shown Figure 2a.11 The photo-

using the MoS2 photoinverter as the imaging pixel as an excellent example of advanced developments in an optoelectronic system based on the 2D semiconductors. Figure 1a,b shows an optical microscopy image and threedimensional (3D) scheme of MoS2 phototransistor. The

Figure 1. (a) Optical microscope image and (b) 3D device scheme of our MoS2 phototransistor. (c) Drain current−gate voltage transfer curve and linear mobility vs gate voltage plot. Figure 2. (a) Schematic view of experimental setup of the phototransistor measurement. (b) Dark and photoinduced transfer curves of the MoS2 phototransistor. (c) The responsivity as a function of applied gate voltage and (d) spectral responsivity at different gate voltages.

multilayer MoS2 nanosheet (∼15 nm) was mechanically exfoliated and transferred on a 285 nm thick SiO2/p+-Si substrate by polydimethylsiloxane (PDMS) stamp. Two graphene nanoflakes for S/D electrodes were aligned and imprinted on top of the MoS2 nanosheet by using our customized direct imprinting system.29 According to Figure 1b, the MoS2 photoactive layer area and width/length (W/L) ratio were estimated to be 140.5 μm2 and 1.7, respectively. The details of device fabrication and characterization are presented later in the paper in the experimental methods. Figure 1c shows the drain current−gate voltage (IDS−VGS) transfer characteristics and gate voltage-dependent linear mobility curve of our MoS2 phototransistor. The linear mobility can be plotted and calculated by the following equations: gm(VGS) = μ lin (VGS) =

∂IDS ∂VGS

induced transfer curves were obtained under the illumination of monochromatic lights with the spectral range of 470−800 nm. The incident optical power on the sample at each wavelength was measured with a UV enhanced silicon photodetector (Newport, 918D-UV-OD3) and an optical power meter (Newport, 842-PE (R2)). The illumination intensities in this spectral range were measured in the range from 0.05 to 0.13 mW cm−2. Figure 2b shows the dark and photoinduced transfer curves of our MoS2 phototransistor exhibiting photoresponses in the 400−700 nm range at a fixed drain voltage VDS = 1 V. The device hardly responded to the near-infrared (NIR) light at 800 nm, consistent with the absorption spectra of a multilayer MoS2 in other literature.32 Responsivity is given by the equation R = Iph/P, where Iph is the photocurrent, and P is the incident optical power illuminating the sample area. Such incident optical power illuminating the sample area is given by the equation P = Ptotal Adevice/Aspot, where Ptotal is the total laser power, Adevice is the area of the device, and Aspot is the area of the illumination light spot that is measured to be ∼3.14 mm2 (the diameter of light spot is 2 mm). Responsivities of our MoS2 phototransistor can be directly extracted from such photoinduced transfer curves (Figure 2b) using above equations, and then they are plotted in Figure 2c. Generally, the responsivity in a phototransistor depends on the VGS. In our MoS2 phototransistor, the responsivities increased with the VGS due to an increase of photocurrent, reaching the maximum at VGS = 20 V (device ON state). Similar phenomena have been also observed in 2D TMDs-based phototransistors.15,28 Spectral responsivities at different gate voltages of VGS = 20 V (device ON state), VGS = −1.6 V (device turn-on state), and VGS = −20 V (device OFF state) were plotted in Figure 2d. The maximum values (device ON state) were in the order of 1 × 104 A/W in the 470−600 nm range. In the turn-on and OFF states, the device exhibited spectral responsivity values in the order of 1 × 100 and 1 × 10−1 A/W, respectively. Considering the device operation

(1)

gm(VGS) Cox ·VDS

(2)

where gm(VGS) is the VGS-dependent transconductance, and Cox is the capacitance density of the gate dielectric layer. The maximum liner mobility (μlin) was estimated to be 34.5 cm2/ (Vs), and a high on/off current ratio of over 1 × 105 was observed at a drain voltage (VD) of 1 V. Considering that the conventional SiO2 dielectric (285 nm) was used as the gate dielectric layer in our device, the electron mobility was relatively high, which could be correlated with a graphene S/D contact. The electrode contact generally has a significant impact on performance of 2D vdW materials-based devices. In our previous work and relevant literature, the Fermi level of graphene could be modulated with an applied bias, indicating that the work function of graphene could be tuned using the back-gate bias.8,30,31 The gate-voltage-induced work function modulation in graphene S/D contact clearly led to the relatively high electron mobility in our MoS2 phototransistor. To investigate the phototransistor characteristics, the photoinduced transfer characteristics were measured using a customized measurement system, which consists of a Hg(Xe) arc lamp as the light source, a grating monochromator, an B

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Figure 3. (a) Dark and photoinduced transfer curve under green LED (λ = 520 nm) illumination. (b) Photoswitching current dynamics at a fixed gate voltage VGS = 0 V under LED switching on/off for a period of 0.1 s (10 Hz). (c) Magnified photoswitching current dynamics of (b). (d) Dark and photoinduced voltage transfer characteristics of the photoinverter. (e) Photoswitching voltage dynamics at a fixed input voltage Vin = −3 V under LED switching on/off for a period of 0.1 s (10 Hz). (f) Magnified photoswitching voltage dynamics of (d).

states and optical power intensities (∼0.1 mW/cm2), the responsivity values of our MoS2 phototransistor (1 × 104 A/W in the ON state and 1 × 10−1 A/W in the OFF state) were reasonably high and comparable to those of 2D TMDs-based phototransistors.6,12,15,18,23,28 The responsivity could be affected by several factors: absorbance of photoactive materials, charge collection efficiency (related with mobility and contact resistance), and so on. As compared with previous reported responsivity values from 2D TMDs-based phototransistors, the comparably high values of 1 × 104 A/W in our device are likely to be related to charge collection efficiency. In our previous study,30 graphene contact was superior to Ti/Au metal contact, resulting in higher performance. We deduce that one of key factors to reach the high responsivity over 1 × 104 is enhanced contact property due to graphene S/D electrode. To examine the feasibility of our MoS2 phototransistor in a more sophisticated optoelectronic system beyond unit a simple unit device, a photoinverter (NOT logic gate) was implemented by connecting a unit phototransistor to an external load resistor (RL = 100 MΩ). A commercial green light-emitting diode (LED) with a wavelength of 520 nm was used as light source to measure the photoinduced static and dynamic behaviors of the photoinverter. Before the photoinverter experiment, the photoinduced static and dynamic characteristics of the unit device were evaluated, as shown in Figure 3a,b. In comparison with the previous results as shown in Figure 2a, more pronounced negative threshold voltage shift and increased photocurrent were observed under green LED illumination (see Figure 3a), because LED optical power of 9.5 mW/cm2 was 100 times higher than that of the Hg(Xe) arc lamp. However, after the LED was turned off (redark condition), the transfer curve had not fully recovered due to the persistent photocurrent (PPC) as previously reported in several literature reports.12,33,34 Such PPC was not serious in our MoS2 phototransistor, but it may possibly be further enhanced by an additional positive gate voltage pulse.12 In

addition to the static behavior, a photocurrent switching behavior was clearly observed, as the LED controlled by a function generator switched on/off for a period of 0.1 s (10 Hz; see Figure 3b). Following the unit device measurement, the characteristics of the photoinverter were examined. Figure 3d shows the voltage transfer characteristics (VTCs) of the photoinverter. As expected, the photoinduced threshold voltage shift of the MoS2 phototransistor produced pronounced output voltage signals under green LED illumination. While the initial switching threshold voltage (VM) was ∼0 V, the photoinduced VM was −5 V under illumination. After the LED was turned off, VM was back to −2 V. The photoinverter also exhibited clear photoswitching voltage dynamics at a constant VIN of −3 V and VDD of 1 V under periodic green LED illumination at 10 Hz, as shown in Figure 3e. According to above photoswitching results (Figure 3c and Figure 3f), the rising and falling times of our MoS2 phototransistor and photoinverter were estimated to be less than 20 ms. To examine reproducibility of MoS2 phototransistors, and photoinverters, four more batches (M1, M2, M3, and M4) were fabricated. These four batches showed similar electrical performance: maximum liner mobility values of 39.6, 33.5, 38.6, 35.4 cm2/(Vs) and on/off current ratios over 1 × 105 (see Figure S1 in the Supporting Information). Phototransistor behaviors were also characterized. A commercial green laser diode (LD) with a wavelength of 532 nm was used as light source. Figure S2a−d exhibited the dark and photoinduced transfer curves of four batches under illuminations with different optical power densities. Photocurrent and responsivity as a function of illumination intensities were plotted in Figure S2e−l in the Supporting Information. Photocurrents monotonously increased with increasing illumination intensity. In contrast, responsivities increased with decreasing illumination intensity, reaching 1 × 103 to 1 × 104 A/W (the device ON state, VG = 20 V) at optical power intensity of 0.1 mW/ cm2. Such maximum responsivity values consistent with C

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previous results are shown in Figure 2d. In addition, the photocurrent switching behavior of M1 device was examined. The rising and decaying times were estimated to be 16 and 1 ms, respectively (see Figure S3 in the Supporting Information). We also implemented four more resistive-load photoinverters, where the output photovoltage signals were clearly extracted both statically and dynamically (see Figure S4 in the Supporting Information). On the basis of electrical performance, responsivity, and photoinverter results, we confirmed the reproducibility of our MoS2 phototransistors. With the photoinverter as the imaging pixel, a prototype visible imager has been demonstrated. Figure 4a displays a

Figure 5. (a) Schematic illustration of the image scanning system with three different light sources: red, green, and blue. (b) Original image made from shadow masks and (c) corresponding image acquired from the MoS2-based single-pixel imager.

the 400−700 nm range reached the responsivity of over 1 × 104 A/W. Demonstrating applicational capability beyond a simple unit photodetector, we have implemented a resistiveload photoinverter (NOT logic gate), where the output photovoltage signals were clearly extracted both statically and dynamically. Finally, using the MoS2 photoinverter as the imaging pixel, a prototype visible imager under green light has been demonstrated. We thus believe that our experimental presentation paves the way for the further developments in an optoelectronic system based on the 2D semiconductors.

Figure 4. (a) Schematic illustration of the image scanning system in a transmissive configuration. (b) Result image acquired from the MoS2based single-pixel imager.

schematic illustration of the biaxial image mechanical scanning system. The green LED was placed as the light source behind an overhead projector (OHP) shadow mask film, where “cat” image was imprinted. On the other side, the image pixel was attached on an x−y planar moving stage, which was placed in proximity to the OHP image mask and was controlled by a stepping motor controller. To acquire the output image, our single-pixel imager scanned the original image in the x- and ydirections for a period of 1 s (1 Hz). The scan ranges in x- and y-directions were both 12.8 mm, with an increment of 0.1 mm, resulting in a pixel resolution of 128 × 128. A semiconductor parameter analyzer was used to measure the output photovoltage signals. The entire image scanning procedure was programmed with LabVIEW. Figure 4b shows the resulting image acquired from the image scanning. The output image clearly displayed that of a cat. In addition to single color imaging, three primary colors (red, green, and blue) imaging was also successfully implemented, as shown in Figure 5. Three color output images of “PHO” (red color), “TON” (green color), and “ICS” (blue color) were clearly observed. To the best of our knowledge, this was the first demonstration of multicolor MoS2-based image sensor. It is worth noting that such a single-pixel imager could be implemented to build a multiple-pixel imager. Once a wafer-scale uniform MoS2 atomic layer film is prepared and signal processing algorithm for the color image is developed, an integrated full-color multipixel array image sensor, like the one in a camera, should be conceivable in principle. In summary, we have fabricated a multilayer MoS 2 phototransistor with graphene S/D electrodes that showed reasonably high electron mobility (maximum: 34.5 cm2/(Vs)) with a conventional SiO2 gate dielectric. Its photoresponse in

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EXPERIMENTAL METHODS Device Fabrication. We prepared a clean, 285 nm thick SiO2/p+-Si wafer as a substrate for estimating the thickness of MoS2 as well as for fabricating the MoS2 phototransistor. PDMS was used as the stamp to exfoliate and transfer the MoS2 nanosheet for the channel and graphene flakes for the S/ D electrodes onto the substrate. The graphene S/D electrodes were then connected to Au/Ti lead lines prepared by using direct-current (DC) sputtering and patterned by a conventional lift-off process. According to the optical microscope image, the thickness of MoS2 was estimated to be ∼15 nm (multilayers), and the W/L ratio was measured to be ∼1.7. Device Characterization. All static electrical and photoelectric measurements of the MoS2 phototransistor and inverter were performed with a semiconductor parameter analyzer (Agilent 4155 C) in dark and under illumination: (1) a light source of 500 W Hg(Xe) arc lamp, a gratings monochromator, and an optical fiber (core diameter of 200 μm) and (2) green LED (520 nm). Dynamic ON/OFF photoswitching behaviors were conducted using a function generator (Tekronix AFG3022B) to operate the LEDs. The biaxial image scanning system consisted of an x−y planar moving stage (VEXTA C7214-9015-1, Oriental Motor), stepping motor controller (D250, Suruga Seiki), and measurement units (PXI-4132 of PXI-1033, National Instruments). Details about the image scanning systems are described elsewhere.35 The system was operated by a customized LabVIEW program. D

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two-dimensional materials and hybrid systems. Nat. Nanotechnol. 2014, 9, 780−793. (9) Yin, Z. Y.; Li, H.; Jiang, L.; Shi, Y. M.; Sun, Y. H.; Lu, G.; Zhang, Q.; Chen, X. D.; Zhang, H.; et al. Single-Layer MoS2 Phototransistors. ACS Nano 2012, 6, 74−80. (10) Choi, W.; Cho, M. Y.; Konar, A.; Lee, J. H.; Cha, G. B.; Hong, S. C.; Kim, S.; Kim, J.; Jena, D.; Joo, J.; et al. High-Detectivity Multilayer MoS2 Phototransistors with Spectral Response from Ultraviolet to Infrared. Adv. Mater. 2012, 24, 5832−5836. (11) Lee, H. S.; Min, S. W.; Chang, Y. G.; Park, M. K.; Nam, T.; Kim, H.; Kim, J. H.; Ryu, S.; Im, S. MoS2 Nanosheet Phototransistors with Thickness-Modulated Optical Energy Gap. Nano Lett. 2012, 12, 3695. (12) Lopez-Sanchez, O.; Lembke, D.; Kayci, M.; Radenovic, A.; Kis, A. Ultrasensitive photodetectors based on monolayer MoS2. Nat. Nanotechnol. 2013, 8, 497−501. (13) Lee, Y. T.; Lee, J.; Ju, H.; Lim, J. A.; Yi, Y.; Choi, W. K.; Hwang, D. K.; Im, S. Nonvolatile Charge Injection Memory Based on Black Phosphorous 2D Nanosheets for Charge Trapping and Active Channel Layers. Adv. Funct. Mater. 2016, 26, 5701−5707. (14) Tosun, M.; Chuang, S.; Fang, H.; Sachid, A. B.; Hettick, M.; Lin, Y.; Zeng, Y.; Javey, A. High-Gain Inverters Based on WSe2 Complementary Field-Effect Transistors. ACS Nano 2014, 8, 4948− 4953. (15) Zhao, Y. D.; Qiao, J. S.; Yu, Z. H.; Yu, P.; Xu, K.; Lau, S. P.; Zhou, W.; Liu, Z.; Wang, X. R.; Ji, W.; Chai, Y. High-ElectronMobility and Air-Stable 2D Layered PtSe2 FETs. Adv. Mater. 2017, 29, 1604230. (16) Wang, Y. L.; Li, L. F.; Yao, W.; Song, S. R.; Sun, J. T.; Pan, J. B.; Ren, X.; Li, C.; Okunishi, E.; Wang, Y. Q.; Wang, E. Y.; Shao, Y.; Zhang, Y. Y.; Yang, H. T.; Schwier, E. F.; Iwasawa, H.; Shimada, K.; Taniguchi, M.; Cheng, Z. H.; Zhou, S. Y.; Du, S. X.; Pennycook, S. J.; Pantelides, S. T.; Gao, H. J. Monolayer PtSe2, a New Semiconducting Transition-Metal-Dichalcogenide, Epitaxially Grown by Direct Selenization of Pt. Nano Lett. 2015, 15, 4013−4018. (17) Huang, Y.; Sutter, E.; Sadowski, J. T.; Cotlet, M.; Monti, O. L. A.; Racke, D. A.; Neupane, M. R.; Wickramaratne, D.; Lake, R. K.; Parkinson, B. A.; Sutter, P. Tin Disulfide-An Emerging Layered Metal Dichalcogenide Semiconductor: Materials Properties and Device Characteristics. ACS Nano 2014, 8, 10743−10755. (18) Yu, P.; Yu, X. C.; Lu, W. L.; Lin, H.; Sun, L. F.; Du, K. Z.; Liu, F. C.; Fu, W.; Zeng, Q. S.; Shen, Z. X.; Jin, C. H.; Wang, Q. J.; Liu, Z. Fast Photoresponse from 1T Tin Diselenide Atomic Layers. Adv. Funct. Mater. 2016, 26, 137−145. (19) Li, L.; Yu, Y.; Ye, G. J.; Ge, Q.; Ou, X.; Wu, H.; Feng, D.; Chen, X. H.; Zhang, Y. Black phosphorus field-effect transistors. Nat. Nanotechnol. 2014, 9, 372−377. (20) Lee, Y. T.; Kwon, H.; Kim, J. S.; Kim, H.-H.; Lee, Y. J.; Lim, J. A.; Song, Y.-W.; Yi, Y.; Choi, W.-K.; Hwang, D. K.; Im, S. Nonvolatile Ferroelectric Memory Circuit Using Black Phosphorus NanosheetBased Field-Effect Transistors with P(VDF-TrFE) Polymer. ACS Nano 2015, 9, 10394−10401. (21) Wachter, S.; Polyushkin, D. K.; Bethge, O.; Mueller, T. A microprocessor based on a two-dimensional semiconductor. Nat. Commun. 2017, 8, 14948. (22) Radisavljevic, B.; Whitwick, M. B.; Kis, A. Small-signal amplifier based on single-layer MoS2. Appl. Phys. Lett. 2012, 101, 043103. (23) Li, X. B.; Wu, J. X.; Mao, N. N.; Zhang, J.; Lei, Z. B.; Liu, Z. H.; Xu, H. A self-powered graphene-MoS2 hybrid phototransistor with fast response rate and high on-off ratio. Carbon 2015, 92, 126−132. (24) Pang, Y. K.; Xue, F.; Wang, L. F.; Chen, J.; Luo, J. J.; Jiang, T.; Zhang, C.; Wang, Z. L. Tribotronic Enhanced Photoresponsivity of a MoS2 Phototransistor. Advanced Science 2016, 3, 1500419. (25) Zhang, Q.; Bao, W. Z.; Gong, A.; Gong, T.; Ma, D. K.; Wan, J. Y.; Dai, J. Q.; Munday, J. N.; He, J. H.; Hu, L. B.; Zhang, D. H. A highly sensitive, highly transparent, gel-gated MoS2 phototransistor on biodegradable nanopaper. Nanoscale 2016, 8, 14237−14242.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.8b01049. Electrical and photodetecting behaviors of four more batches of MoS2 phototransistors and photoinverter characteristics (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (M.-C.P.) *E-mail:[email protected]. (D.-K.H.) ORCID

Young Tack Lee: 0000-0003-4502-5167 Byeong-Kwon Ju: 0000-0002-5117-2887 Seongil Im: 0000-0003-0723-5075 Do Kyung Hwang: 0000-0001-8736-9262 Author Contributions #

Authors with equal contribution.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.K.H. acknowledges the financial support from the Korea Institute of Science and Technology (KIST) Institution Program (Grant Nos. 2E28180 and 2E28200) and the National Research Foundation of Korea (NRF) (Grant No. 2017R1A2B2005640). S.I. acknowledges the financial support from NRF (SRC program: Grant No. 2017R1A5A1014862, vdWMRC center). This research was supported by Basic Science Research Program through the NRF funded by the Ministry of Education (2010-0020163) (Y.T.L.). M.C.P. acknowledges the financial support from the Ministry of Culture, Sports and Tourism (MCST) and the Korea Creative Content Agency (KOCCA) in the Culture Technology (CT) Research & Development Program 2018 (R2017060005, Development of AR Platform based on Hologram).

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