Fluorescence “Giant” Red Edge Effect
6.5 Photodetector applications of carbon quantum dots and graphene quantum dots
Based on the sample geometry, CQDs/GQDs-based photodetectors can be classified into following three types of devices: (1) field effect transistor (FET)-based photo- detectors, (2) CQDs or GQDs-sensitized nanomaterial-based UV and broadband photodetectors, and (3) polymer nanocomposite-based photodetectors. Depending upon photoabsorption, two major photodetectors were reported, namely UV photo- detectors and broadband photodetectors.
Figure 6.9 (A) Schematic, (B) top-view SEM, (C) cross-section SEM image of Si/GQDs heterojunction solar cell. (D) IV characteristics of Si/GQD solar cell. (E) Schematic of GQD-based LED structure with energy band diagram. (F) Electroluminescent image of GQD-based LEDs.
Reprinted with permission from S. H. Song, M.-H. Jang, J. Chung, S. H. Jin, B. H. Kim, S.-H. Hur, et al., Highly efficient light-emitting diode of graphene quantum dots fabricated from graphite intercalation compounds, Advanced Optical Materials 11 (2014) 1016 1023.
M.L. Tsai, W.C. Tu, L. Tang, T.C. Wei, W.-R. Wei, S.P. Lau, et al., Efficiency enhancement of silicon heterojunction solar cells via photon management using graphene quantum dot as downconverters, Nano Letters 1 (2016) 309 313. Copyright 2016, American Chemical Society. Copyright 2014, Wiley-VCH Verlag GmbH & Co.
6.5.1 FET-based photodetectors using carbon quantum dots and graphene quantum dots
FET-based photodetectors were studied by dispersing a thin layer of GQDs on insu- lating substrates like glass, quartz, SiO2/Si, etc. Some researchers fabricated photo- detector devices after depositing GQDs on graphene and transition metal dichalcogenide (TMD) films, which acted as fast electron transport layer due to their 2D geometry. However, in both types of devices, electrical contacts were made laterally using metal electrodes. Zhang et al. fabricated DUV photodetectors with fast-response time and high-sensitivity[64]. The photodetector device was fab- ricated after deposition of Ag and Au asymmetric electrodes on quartz substrates.
Solution processed GQDs of size 4 5 nm and thickness 0.4 2.5 nm consisting of 1 4 layers of graphene were deposited on the electrodes.Fig. 6.10Arepresents the schematic diagram of device structure. Under DUV light, excitation carrier excited to LUMO of the GQDs and transported one GQD to another via hopping. Carrier transport mechanism in asymmetric Ag-Au electrodes under illumination is shown inFig. 6.10B. Even after illumination of 254 nm UV light with weak intensity of 42μW/cm2, their device showed on/off ratio ofB6000. The response and recovery
(A) (B)
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Figure 6.10 (A) Schematic device structure of DUV photodetector. (B) Energy band diagram and carrier transport mechanism in asymmetric Ag-Au electrodes. (C) Transient photoresponse of the device at various intensities of 254 nm UV light. (D) Rise and decay curve of transient photoresponse. (E) Responsivity plot of the photodetector at15 V bias.
The photocurrent variation with light intensity at15 V bias is presented at inset.
Reprinted with permission from Q. Zhang, J. Jie, S. Diao, Z. Shao, Q. Zhang, L. Wang, et al., Solution-processed graphene quantum dot deep-UV photodetectors, ACS Nano 2 (2015) 1561 1570. Copyright 2015, American Chemical Society.
time of the device were 64 Ms and 43 Ms, respectively.Fig. 6.10Cshows transient photoresponse of the DUV device under various intensities (15 42 mW/cm2) of 254 nm UV light in vacuum condition at 15 V bias. The estimation of rise and decay time from the transient photoresponse is presented inFig. 6.10D. The photo- detector demonstrated responsivity of 2.10 mA/W (Fig. 6.10E).
Materials that absorb light from spectral range covering DUV, visible, and near- infrared regions are attractive for broadband photodetectors. Such materials can also be applied in broadband modulators, solar cells, bioimaging, and optical fiber communications. Several research work have been conducted on broadband photo- detection using a combination of low- and high-band gap semiconductors.
However, fabrication of such detector using a single material is very much attrac- tive due to simple device configuration. Tang et al. fabricated broadband photode- tector in the wavelength range 300 1000 nm using chemically grown NGQDs[65].
The broadband photoabsorption of the detector was attributed to the layered struc- ture of the NGQDs.Fig. 6.11Arepresents the schematic fabrication process of the device using photolithography. The photodetector device was fabricated by drop coating NGQDs on interdigitated gold electrodes (Fig. 6.11A). The photodetector under bias condition is presented inFig. 6.11B. The photoresponse of NGQDs was monitored under different monochromatic light illuminations (Fig. 6.11C). The maximum responsivity of the detector was 325 V/W at 405 nm.
Tetsuka et al. synthesized o-phenylenediamine (OPD-GQDs) and diamino- naphthalene (DAN-GQDs) functionalized NGQDs with tailored optical properties and studied photodetection using graphene/NGQDs hybrid phototransistors [66].
The NGQD was deposited on the top of a graphene-based field-effect transistor.
Fig. 6.12A shows the schematic representation of the photodetector, where p-Si with 90 nm SiO2 layer was used as substrate, graphene used as carrier transport Figure 6.11 (A) Schematic representation of the photodetector fabrication process. (B) The schematic device structure of the photodetector under bias. (C) The transient photoresponse of NGQDs-based photodetector with various light sources and bias currents.
Reprinted with permission from L. Tang, R. Ji, X. Li, G. Bai, C.P. Liu, J. Hao, et al., Deep ultraviolet to near-infrared emission and photoresponse in layered N-doped graphene quantum dots, ACS Nano 6 (2014) 6312 6320. Copyright 2014, American Chemical Society.
layer, and NGQDs used as photon absorption layer. The actual digital photograph of graphene/NGQDs phototransistor is shown inFig. 6.12B.Fig. 6.12C illustrates the charge transfer mechanism from NGQDs to graphene layer under illumination.
The temporal photoresponse under illumination of 440 nm laser light with intensity 1.2μW is shown in Fig. 6.12D. The change in photocurrent with time under the Figure 6.12 (A) Graphic diagram of a graphene/NGQDs FET-based phototransistor.
(B) Digital photographic images of the devices. (C) Carrier transport mechanism at graphene/
NGQDs junction under illumination. (D) Photocurrent response of the photodetector.
(E) Variation of the response time with the excitation power of 25 to 970 nW. (F) Change in responsivity and detectivity with optical power (laser wavelengths 440 785 nm). (G) The on and off photocurrent response of the OPD-GQDs phototransistor without any graphene layer.
Temporal photocurrent response is presented at inset.
Reprinted with permission from H. Tetsuka, A. Nagoya, T. Fukusumi, and T. Matsui, Molecularly designed, nitrogen-functionalized graphene quantum dots for optoelectronic devices, Advanced Materials 28 (2016) 4632 4638. Copyright 2016, Wiley-VCH Verlag GmbH & Co.
excitation power varied from 25 to 970 nW for graphene/OPD-GQDs phototransis- tor is shown inFig. 6.12E.Fig. 6.12Fshows photoresponsivity and detectivity plots under different illumination power (laser wavelengths 440 785 nm). The photocur- rent response under on/off condition of the phototransistor without any graphene layer is presented in Fig. 6.12G. Temporal photocurrent response is presented at inset. Graphene/NGQDs phototransistor demonstrated high photoconductive gain of 231010. Photoresponsivity and detectivity of the device increase linearly with the decrease of power of the incident laser of power 1.5 nW. The maximum photorespon- sivity and detectivity were B3.53104A/W and B1.231012J, respectively. The same group also improved the device performance using a boron nitride nanosheets buffer layer for efficient charge separation, and transport of photogenerated carriers from NGQDs. The photodetector demonstrated photoresponsivity and detectivity of 2.33106A/W and 5.531013 Jones, respectively, in the DUV range without any back-gate voltage[67]. Kim et al. studied graphene/GQD/graphene-based photodetec- tor, which demonstrated strong responsivity in UV-Vis-NIR range[68]. Such GQD sandwiched between graphene layers photodetector showed detectivity .1011cm Hz1/2/W and responsivity 0.2 0.5 A/W in the UV to near infrared region.
Recently, 2D TMD materials such as MoS2, WS2, MoSe2, WSe2,and MoTe2have attracted much attention for photodetector application for their high carrier mobility, excellent tuneable band gap, and high ON/OFF ratio. But due to their 2D structure and atomically thin layers, the photoabsortion and emission are very limited. Therefore, the combination of 2D materials and QDs is particularly attractive for photodetector devices. As QDs are excellent photoabsorbers, they can inject photogenerated carriers into the 2D layers of TMDs efficiently, and the photogenerated carriers can be trans- ported to the electrodes very fast due to the superior carrier mobility of 2D structures.
Nguyen et al. reported a hybrid nanostructure consisting of monolayer (ML) WSe2covered with NGQDs (WSe2/NGQDs) thin films[69].Fig. 6.13A and Brep- resent the schematic diagram of ML WSe2FET device and optical photograph of ML WSe2/NGQDs devices, respectively. Gate voltage characteristics of both devices at VDS51 V are shown inFig. 6.13C.Fig. 6.13D shows schematic view of ML WSe2/NGQD under light. Time-dependent photocurrent response of both devices is shown inFig. 6.13E. FET characteristic transfer curves under dark and light illumination condition and along with different intensities of lights are pre- sented inFig. 6.13F and G. Photoresponsivity of WSe2/N-GQD photodetector was reported to be about 480% higher compared to WSe2-based photodetector. Such high responsivity is accredited to the charge transfer from NGQDs to WSe2. Responsivity plots as a function of back-gate voltage and power density of ML WSe2/N-GQD photodetector are shown inFig. 6.13H and I, respectively. The high- est photoresponsivity was found to be 2578 A/W at power density of 1.9μW/cm2. The rise and decay time of the photodetector was found to be smaller than 0.5 s (Fig. 6.13J). Change in responsivity with back-gate voltage and bias voltage of ML WSe2/N-GQD under light of wavelengthB405 nm is presented inFig. 6.13K. It is reported that the responsivity of the WSe2/NGQD photodetector is stable under ambient experimental condition up to 30 days. Chen et al. reported the photodetec- tor properties of GQDs deposited MoS2 on thin film based phototransistors [70].
The phototransistor devices were fabricated on SiO2/Si substrates as shown in Fig. 6.14A. MoS2-GQDs device was fabricated after drop casting of GQDs solution on the MoS2 layer followed by heat treatment at 70C. The attachment of GQDs onto MoS2 film creates an n-n type van der Waals heterostucture, which Figure 6.13 (A) Schematic image of monolayer (ML) WSe2FET device. (B) Optical microscopy image of ML WSe2FET and ML WSe2/NGQDs devices. (C) Gate voltage characteristics atVDS51 V. (D) Schematic view of ML WSe2/NGQD under light. (E) Time- dependent photocurrent response of both devices at VDS51 V and VG50 V. Transfer curves of ML WSe2and ML WSe2/N-GQD devices (F) under dark and light and (G) under illumination of different intensities of lights. (H) Responsivity plots vs back-gate voltage plots of the ML WSe2/N-GQD photodetector at various intensities. (I) Responsivity plots vs power density plots of ML WSe2/N-GQD photodetector. (J) Transient photocurrent of ML WSe2/N-GQD photodetector under different bias voltage. (K) Change in responsivity with gate and bias voltage of ML WSe2/N-GQD under light of wavelengthB405 nm.
Reprinted with permission from N.D. Nguyen, M.O. Hye, T.D. Ngoc, S. Bang, S.J. Yoon, and M.S. Jeong, Highly enhanced photoresponsivity of a monolayer WSe2 photodetector with nitrogen-doped graphene quantum dots, ACS Applied Materials & Interfaces 10 (2018) 10322 10329. Copyright 2018, American Chemical Society.
significantly improved the carrier mobility through the injection of photogenerated electrons from GQDs to MoS2.Fig. 6.14B shows the transfer curves of MoS2and MoS2-GQDs devices with source-drain voltage (Vds)B1 V. The drain current (Id) vs source-drain voltage (Vds) of both devices under light and dark is plotted in
Light Source Graphene quantum dots Source
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Figure 6.14 (A) Schematic device structure of a MoS2/GQDs phototransistor. (B) Transfer curves of MoS2and MoS2-GQDs devices with source-drain voltage: Vds51 V. (C) Drain current (Id) vs source-drain voltage (Vds) under light and dark. Laser wavelengthB405 nm with power B17μW. (D) Temporal photoresponse of MoS2and MoS2/GQDs devices. (E) Variation of photocurrent vs gate voltage for both device under light of powerB30.1μW. (F) Variation photoresponsivity with incident light power at different back-gate voltage.
Reprinted with permission from C. Chen, H. Qiao, S. Lin, C.M. Luk, Y. Liu, Z. Xu, et al., Highly responsive MoS2 photodetectors enhanced by graphene quantum dots, Scientific Reports 5 (2015) 11830. Copyright 2015, Springer Nature.
Fig. 6.14C. After deposition of GQDs, the rise time of the photodetector was improved from 20 s to 70 Ms (Fig. 6.14D). The MoS2-GQDs phototransistor device also demonstrated superior gate voltage dependent photocurrent under incident powerB30.1μW (laser wavelengthB405 nm) (Fig. 6.14E).
The device demonstrated photoresponsivity of 1.63104/AW with a photogain of 2.43107(Fig. 6.14F). The improvement of photodetection in MoS2-GQDs pho- totransistor was explained on the basis of tunneling effect, the re-absorption of photons, and holes transfer from GQDs to MoS2. Sun et al. fabricated heterojunc- tion photodetector using p-type monolayer of WSe2 and n-type Si substrate [71].
The WSe2 layer was deposited by physical vapor deposition. The photodetector properties were elevated by incorporating GQDs on the WSe2/Si heterojunction by drop coating of GQDs solutions. The GQDs/WSe2/Si heterojunction showed responsivity of B707 mA/W and detectivity of B 4.513109 Jones. The device demonstrated response time of 0.2 Ms [71]. The superior photodetector perfor- mance of WSe2/Si heterojunction after introduction of GQDs is attributed to strong light absorption and long carrier lifetimes. Wu et al. studied N-CQDs/graphene hybrid composites for UV photodetector application[72]. Photodetector was fabri- cated by spin-coating NCQDs on single layer graphene film deposited on SiO2/p- type Si substrate. Negative responsivity up to 2.53104/AW was observed under UV light of wavelength 375 nm.
6.5.2 Carbon quantum dots or graphene quantum dots- sensitized nanomaterial-based photodetectors
Several researchers applied GQDs or CQDs as photosensitizers similar to semicon- ductors QDs with nanorods and nanowires for enhancing the photoconducting prop- erties of the device. These 0D carbon materials create favorable band alignment between the QDs and semiconductors and enhance the charge carrier transport to the electrodes. Dhar et al. studied a Schottky junction UV photodetector using ZnO nanorods (ZnO NRs) and conducting polymer poly 3,4-ethylenedioxythiophene:
polystyrene sulfonate (PEDOT:PSS) junction [18,19]. The photodetector properties of such inorganic/organic heterojunction was enhanced after attachment of GQDs with ZnO NRs. Top-view SEM micrographs of ZnO NRs before and after spin coating of PEDOT:PSS are presented inFig. 6.15A and B, respectively. Plane-view TEM micrographs of GQDs and GQDs decorated ZnO NRs are shown in Fig. 6.15C and D, respectively. The absorption and emission spectra are depicted in Fig. 6.15E and F. Due to strong UV absorption, both GQDs and NGQDs are attrac- tive as photosensitizer with metal oxide semiconductors like TiO2, ZnO, In2O3,etc.
To demonstrate the photodetector behaviors of GQDs and NGQDs-sensitized nanor- ods, the following samples were prepared. The studied samples were (1) FTO/ZnO NRs/PEDOT:PSS/Ag (S1), (2) FTO/GQD decorated ZnO NRs/PEDOT:PSS/Ag (S2), and (3) FTO/NGQD decorated ZnO NRs/PEDOT:PSS/Ag (S3). Fig. 6.15G represents theJ-Vcharacteristics of S3 sample at dark condition and the illustration of the device is presented at inset. Under white light illumination, the reverse
saturation current of the device changes rapidly, as shown inFig. 6.15H. Transient photoresponse of all samples under illumination of wavelengthB340 nm of power 0.38 mW/cm2is depicted in Fig. 6.16A. The on/off ratio for S3 detector was found to be three times and 78 times higher compared to S2 and S1 samples, respectively.
Responsivity and detectivity plots of S1, S2, and S3 samples are plotted in Fig. 6.16A C, respectively. The responsivity and detectivity were found maximum at 340 nm wavelength for all samples. The responsivity values for S1, S2, and S3 samples are 3, 36, and 158 A/W, respectively. The detectivity values of above sam- ples are 3.8131011, 1.7231012, and 8.7531012Hz1/2/W, respectively. It is obvi- ous that NGQD modified detector demonstrated higher photocurrent, responsivity, and detectivity in comparison with GQD modified and pristine ZnO NR detectors due to strong UV absorption of NGQDs. The photodetector performance of S3 sam- ple was further improved by increasing the electrical conductivity of PEDOT:PSS Figure 6.15 SEM image of ZnO nanorods (A) before and (B) after spin coating of PEDOT:
PSS layer. (C) Plane-view TEM image of GQDs. Inset: HRTEM image of a GQD. (D) TEM image of GQDs coated ZnO NRs. Inset: HRTEM image of ZnO nanorods. Absorption and PL spectra of (E) GQDs and (F) NGQDs. Insets: Digital photographs of GQD and NGQD solutions. (G) J-V plot of S3 at dark condition. Inset: Schematic diagram of UV detector (S3 sample). (H) J-V plot in semilogarithmic scale of sample S3 under dark and light of intensity 80 mW/cm2.
Reprinted with permission from S. Dhar, T. Majumder, S.P. Mondal, Graphene Quantum Dot-Sensitized ZnO Nanorod/Polymer Schottky Junction UV Detector with Superior External Quantum Efficiency, Detectivity, and Responsivity, ACS App. Mater. Interfaces 8 (2016) 31822 31831. Copyright 2016, American Chemical Society. S. Dhar, T. Majumder, and S.P.
Mondal. Phenomenal improvement of external quantum efficiency, detectivity and
responsivity of nitrogen doped graphene quantum dot decorated zinc oxide nanorod/polymer Schottky junction UV detector, Materials Research Bulletin 95 (2017) 198 203. Copyright 2016, Elsevier; T. Majumder, S.P. Mondal, Advantages of nitrogen-doped graphene quantum dots as a green sensitizer with ZnO nanorod based photoanodes for solar energy conversion, Journal of Electroanalytical Chemistry 769 (2016) 48 52, Copyright 2016, Elsevier.
polymer. The maximum responsivity was B247 A/W at 340 nm wavelength at 1 V bias.
Xie et al. reported self-powered photodetector using CQD coated silicon nano- wire (Si NW) core-shell heterojunction[73]. Vertical Si NW array was prepared by Ag-assisted chemical-etched process. Fig. 6.17A shows the schematic representa- tion of the device structure. The CQD/Si NW core-shell heterojunction demon- strated rectifying nature with rectification ratio of 103at 60.8 V under dark. The power conversion efficiency of the device was found to be 9.10% under AM 1.5 G irradiation. Fig. 6.17B shows IV plots of the sample in dark and under irradiation of light of wavelength 600 nm of intensity 100 mW cm2. The device exhibited high photocurrent upto 0.1 mA at zero bias. The photosensitivity (Ilight/Idark) of the sam- ple was found to be B33103 (Fig. 6.17C). The rise time and fall time were obtained 20 and 40μs, respectively (Fig. 6.17D). The transient photoresponse of Si NW array/CQD heterojunction photodetector under incidence of pulsed light of fre- quencies of 500, 1000, and 2200 Hz is presented inFig. 6.17E. The device demon- strated responsivity of 353 mA/W.
Ghosh et al. fabricated GQD decorated ZnO nanorods based on UV photodetec- tor with high photoresponsivity and detectivity at low applied potential [74]. In their study, GQDs, CQDs, GO, and RGO were prepared using solution growth pro- cess and sensitized with ZnO NRs grown on FTO substrates. UV photodetector properties were studied using all samples, and a comparative study was demon- strated. I-V characteristics of GQD/ZnO NRs at various intensities of 365 nm UV light are presented inFig. 6.18A. The schematic device configuration is also shown at the inset of Fig. 6.18A. Responsivity vs wavelengths plot of optimized GQD/
ZnO NRs device along with control sample is presented inFig. 6.18B. The highest photoresponsivity and detectivity are B6.923104A/W and B1.7831015 Jones, respectively under 10μW illumination of UV light at 2 V bias.
Figure 6.16 (A) Time-dependent photocurrent (J-t plot) of S1, S2, and S3 devices at 1 V bias and under UV light of wavelength 340 nm. (B) Responsivity and (B) detectivity plots for S1, S2, and S3 samples at 1 V bias.
Reprinted with permission from S. Dhar, T. Majumder, and S.P. Mondal. Phenomenal improvement of external quantum efficiency, detectivity and responsivity of nitrogen doped graphene quantum dot decorated zinc oxide nanorod/polymer Schottky junction UV detector, Materials Research Bulletin 95 (2017) 198 203. Copyright 2016, Elsevier.
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Figure 6.17 (A) Schematic diagram of the Si nanowire/CQD device. (B) I-V characteristics of Si NW array/CQD heterojunction under dark and illumination of light of 600 nm wavelength. (C) Transient photoresponse under on/off light. (D) Calculation of rise time and fall time. (E) Transient photoresponse of Si NW/CQD heterojunction under incident of pulsed light of frequencies of 500, 1000, and 2200 Hz.
Reprinted with permission from C. Xie, B. Nie, L. Zeng, F.X. Liang, M.Z. Wang, L. Luo, et al., Core shell heterojunction of silicon nanowire arrays and carbon quantum dots for photovoltaic devices and self-driven photodetectors, ACS Nano 8 (2014) 4015 4022.
Copyright 2014, American Chemical Society.
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Figure 6.18 (A) IV characteristics of GQD/ZnO NRs photodetector under incident of UV (B365 nm) light of different intensities. The schematic diagram of the device is shown at inset. (B) Responsivity vs wavelength plots of ZnO NRs and GQD/ZnO NRs. The schematic presentation of electron transfer is shown at inset.
Reprinted with permission from D. Ghosh, S. Kapri, and S. Bhattacharyya, Phenomenal ultraviolet photoresponsivity and detectivity of graphene dots immobilized on zinc oxide nanorods. ACS Applied Materials & Interfaces 8 (2016) 35496 35504. Copyright 2016, American Chemical Society.