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Multi-Heterojunction Phototransistors Based on Graphene-PbSe Quantum Dot Hybrids Yating Zhang, Mingxuan Cao, Xiaoxian Song, Jianlong Wang, Yongli Che, Haitao Dai, Xin Ding, Guizhong Zhang, and Jianquan Yao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b07318 • Publication Date (Web): 31 Aug 2015 Downloaded from http://pubs.acs.org on September 5, 2015

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The Journal of Physical Chemistry

Multi-heterojunction Phototransistors Based on Graphene-PbSe Quantum Dot Hybrids Yating Zhang1, 2,∗, Mingxuan Cao1, 2, Xiaoxian Song1, 2, Jianlong Wang, Yongli Che1, 2, Haitao Dai3, Xin Ding1, 2, Guizhong Zhang1, 2, and Jianquan Yao1, 2 1

Institute of Laser & Opto-Electronics, College of Precision Instruments and Opto-electronics Engineering, Tianjin University, Tianjin 300072, China

2

Key Laboratory of Opto-electronics Information Technology (Tianjin University), Ministry of Education, Tianjin 300072, China 3

Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology, School of Science, Tianjin University, Tianjin 300072, China

ABSTRACT Graphene-semiconductor quantum dot (QD) hybrid field effect phototransistors (FEpTs) have attracted much interest due to their ultrahigh gain and responsivity in photo detection. However, most reported results are based on single-layer heterojunction, and the multi-heterojunction FEpTs are often ignored. Here, we design two typical multi-heterojunction FEpTs based on graphene-PbSe quantum dot (QD) hybrids, including QD at the bottom layer (QD-bottom) and

∗

Addressed correspondence to [email protected] 1 ACS Paragon Plus Environment


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graphene at the bottom layer (G-bottom) FEpTs. Through a comparative study, G-bottom FEpTs showed a multi-saturation behavior due to the multi-graphene layer effect, which was absent in the QD-bottom FEpTs. The mobilities for electrons and holes were µ E = 147 cm2 V−1 s−1 and µ H = 137 cm2 V−1 s−1 in the G-bottom FEpTs and µ E = 14 cm2 V−1 s−1 and µ H = 59 cm2 V−1 s−1 in the QD-bottom FEpTs. Higher responsivity (~ 106 A W-1) and faster response rate were both achieved by the G-bottom FEpTs. All of the advantages in G-bottom FEpTs were attributed to the back-gate effect. Therefore, high performance is expected in those FEpTs whose heterojunctions are designed to be close to the back-gate.

KEYWORDS: Field effect phototransistor, graphene-QD hybrids.

1. INTRODUCTION Field effect phototransistors (FEpTs) based on graphene-semiconductor quantum dots (QDs) and their hybrids have achieved ultrahigh performance 1 and are attracting more attention for the use in near infrared (NIR) detectors due to their low cost, low energy-consumption, flexibility, easy fabrication and easy integration

2

. The

achievement of ultrahigh responsivity (i.e., the photo-generated current per incident optical power) and gain (i.e., the ability to provide multiple electrical carriers per single incident photon) is mainly attributed to the large absorbance of QDs, the fast transport in graphene, and the charge carrier transfer between the interface of the heterojunction 3. The large absorbance of QDs and fast carrier transport in graphene are the intrinsic properties of the two materials. Therefore, heterojunction plays a

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critical role in carrier transfer and in photo detection, particularly in these types of devices 4. Recently, we have studied the two typical heterojunctions in FEpTs, including layer-heterojunction (LH) and bulk-heterojunction (BH), and it was found that LH-FEpTs show higher performance than BH-FEpTs in photoresponse 5. The excellent responsivity of these types of FEpTs was also observed and reported by other groups

3, 6

. However, most researchers focused on single-heterojunction FEpTs,

and ignored the multi-heterojunction (MH) devices. In this study, we designed and fabricated two types of multi-heterojunction FEpTs based on graphene and QD hybrid materials that were expected to have higher performance, including FEpTs with QD at the bottom layer (QD-bottom) or graphene at the bottom layer (G-bottom). Both types of FEpTs exhibited good photoresponse in the NIR region and responsivities of up to 105 A W-1. Comparing their photoresponses, the G-bottom FEpTs showed a higher performance in photoresponse time and responsivity due to their strong back-gate effect.

2. EXPERIMENTAL SECTION Materials PbSe QDs were synthesized using a wet chemical method 7, and a typical synthesizing process was detailed in the Supplementary Materials. Graphene powder used in the hybrids was prepared by reducing graphene oxide, which was directly purchased from The Sixth Element, Inc.. Characterization



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PbSe QDs were characterized by optical spectra and a transmission electron microscope (TEM) (FEI, Tecnai G2 F20, Netherlands) at 200 kV (Figure 1 (a) insert, scale bar of 50 nm). The absorbance and photoluminescence (PL) spectra were display from Figure 1 (a), which was analyzed using a PbSe QDs toluene solution and a Zolix Omni-λ300 spectrometer. The absorption peak was located at 1487 nm, while the PL peak was at 1525 nm when excited by a continuous-wave (cw) laser at a wavelength of 532 nm. According to a four-band-envelope-function formulism, the average diameter of PbSe QDs is 6.3 nm 8. Based on the TEM images, the average size of the PbSe QDs was 6.3 nm, consistent with the size deduction in the absorbance spectrum in Figure 1 (a). The transmission peaks of the PbSe QDs were characterized by Fourier transform infrared (FTIR) spectra using a FTIR-650-spectrometer (Tianjin Gangdong sci.&tech. Development Co., Ltd.), as figure 1 (b). The feature of the PbSe QDs was similar to the FITR spectra of ligands, which was attributed to dioctyl phthalate (DOP). As the capping ligand of QDs, it was believed that DOPs were either the by-products of PbSe QD reactions or products of TOP oxidation during storage. Graphene powder was characterized first by FTIR spectrometry. After amplification of the FTIR spectrum (Figure 1 (b), insert), the feature peaks can be observed and are attributed to a non-oxygenated benzene ring (1548 cm-1) and a hydroxyl carbon (1108 cm-1) 9. From a quantitative analysis (see Supplementary Materials), the powder used in the experiment was graphene with impurities of hydroxyl carbons (22.09 %) and carbonyl carbons (7.76 %). Because the two 4 ACS Paragon Plus Environment

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impurities often appear at the edge, instead of the surface, of a graphene sheet, they were assumed to have little influence on the heterojunction. Thus the interface of the LH was attributed to a graphene-DOP-QD structure.

Figure 1. (a) The absorbance (Abs., red line) and photoluminescence (PL, black line) spectra of PbSe QDs; the insert is a TEM image of PbSe QDs with a scale bar of 50 nm. (b) FTIR spectra of PbSe QDs and graphene; the insert is an amplified FTIR spectrum of graphene powder.

Device Fabrication and SEM Images QD-bottom and G-bottom FEpTs based on graphene-PbSe QD hybrids were experimentally fabricated. The schematic diagrams are displayed in figure 2 (a) and (c), respectively. The substrates were Si n+ and SiO2, and the thickness of the SiO2 layer was 300 nm. QD-bottom FEpTs were fabricated using the following processes. The Au source and drain electrodes (200 nm thick) were thermally evaporated through a shadowed mask on the SiO2 layer. Three layers of the PbSe QD and graphene heterojunction 5 ACS Paragon Plus Environment


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were deposited layer by layer. Each layer was prepared as follows. A drop of PbSe QD toluene solution (10 mg mL-1) was first deposited on the spinning substrate at a speed of 2000 rpm and allowed to dry for 15 s. 3 drops of 2 % by volume ethanedithiol (EDT) solution were deposited on the rotating substrate to promote ligand exchange, then 2 drops of acetonitrile and 2 drops of 1 mg ml-1 graphene toluene solution were deposited. G-bottom FEpTs were fabricated using a similar process. The Au source and drain electrodes (200 nm thick) were thermally evaporated through a shadowed mask on the SiO2 layer. Three layers of the graphene-PbSe QD heterojunction were deposited one layer at a time. The first graphene layer was prepared by depositing 2 drops of 1 mg ml-1 graphene toluene solution on the spinning substrate at a speed of 2000 rpm and allowing it to dry for 15 s. Next, a drop of the PbSe layer was deposited on the spinning substrate at a speed of 2000 rpm, 3 drops of 2 % EDT solution were deposited on the rotating substrate for ligand exchange, then 2 drops of acetonitrile and 2 drops of toluene were deposited. Each device was dried in vacuum conditions overnight. The SEM images of cross sections of the QD-bottom and G-bottom FEpTs are shown in figure 2 (b) and (d), respectively. The structure of each device is clearly visible. The bottom layer is Si n+, on which is there is a SiO2 layer with a thickness of 300 nm. The top layer is the MH layer. The thickness of the QD-bottom FEpTs and G-bottom FEpTs were 109 nm and 103 nm, respectively. Clearly, the MH layers in


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the QD-bottom FEpTs were more even than in the QD-bottom FEpTs due to the greater uniformity in the first layer of QDs than in the graphene sheet layer.

Figure 2. Schematic illustration of the QD-bottom FEpTs (a) and the G-bottom FEpTs (c) on a Si n+/SiO2 substrate. Cross-sectional SEM images of the MH channel in the QD-bottom FEpTs (b) and the G-bottom FEpTs (d). Device Measurements For the electrical measurements, a bias voltage (VSD) was applied to the source (i.e., ground connection) and drain electrodes and was measured using a KeithleyTM 2400; the channel current flowing into the drain was denoted by ID and was also measured using the KeithleyTM 2400. A gate voltage (VG) was applied to the gate electrode and



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the ground connection by using an HP6030A. During the electrical measurements, the FEpT was fixed on a holder. The measurements were performed at room temperature in the dark. For the optical measurements, the FEpT was illuminated with different light irradiances (Ee). The light source was a cw laser with a wavelength of 808 nm.

3. RESULTS AND DISCUSSION Electrical properties

Figure 3. Representative output characteristics (ID ~ VSD) of the QD-bottom FEpTs (a) and the G-bottom FEpTs (b) without light illumination at gate voltages (VG) of 0 V, ±1 V, ±3 V, ±5 V, ±7 V, and ±9 V for the QD-bottom FEpTs and 0 V, ±3 V, ±6 V, ±9 V, ±12 V, and ±15 V for the G-bottom FEpTs. Figure 3 (a) and (b) show the output characteristics (ID ~ VSD) of the QD-bottom and G-bottom FEpTs, respectively, at various gate voltages. Ambipolar characteristics for both devices were obvious due to the saturation behavior in both the hole and electron regimes. The difference derives from the single and double depletion regimes present in QD-bottom and G-bottom FEpTs, respectively. According to the basic theory of traditional field effect transistors (FETs), the saturation behavior in the hole or electron regimes is due to the depletion regimes in graphene sheets 10. In G-bottom 8 ACS Paragon Plus Environment

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FEpTs, holes showed a double-depletion regime at low gate voltage, i.e., 0 and 3 V in the first quadrant as shown in figure 3 (b). This is a result of MH, in which each saturation region corresponds to a layer of graphene sheet. Three saturation regions would be observed if the carriers transfer effectively. The absence of a multi-saturation region, i.e., B-bottom FEpTs, indicates the transfer of carriers was ineffective. This feature becomes clearer when transfer characteristics are plotted.

Figure 4. Representative transfer characteristics (ID ~ VG) of the graphene FEpTs, QD-bottom FEpTs and G-bottom FEpTs without light illumination at a bias voltage (VSD) of 0.8 V. For a low bias voltage (0.8 V), transfer characteristics (ID ~ VG) of the graphene, QD-bottom and G-bottom FEpTs are displayed in figure 4. As well known, the symmetry of the transfer curve of pristine graphene FEpTs suggests equal mobilities for holes and electrons, which results in a minimum in the transfer curve, which is referred to as the Daric point 3a. When modified by QDs, such a point shifts slightly for QD-bottom FEpTs and shifts negatively to −1 V for G-bottom FEpTs for which the transfer curve contains another minimum point at −7.5 V. For PbSe QDs, the affinity of electrons is 4.26 eV, the affinity of holes is 5.53 eV 11, and the Fermi level 9 ACS Paragon Plus Environment


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is approximately at 4.9 eV; for grapheme, the Dirac point is located at 4.6 eV

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6a

.

When they come in contact with one another, holes flow into graphene, which becomes p-doped, as shown in the insert of figure 4 3. Various shift voltage ranging from 0 V to 60 V have been reported previously

3a, 5

. The shift degree is closely

related to the properties of the graphene-QD heterojunction, including the surface defects of QDs, the barriers formed by ligands and the degree of energy level matching between graphene and QDs. Though it is complex to analyze such heterojunction properties, it is typically concluded that carriers effectively transfer in FEpTs with a large Dirac point shift. According to the sensing mechanism, the responsivity of graphene-QD hybrid FEpTs strongly depends on the carrier transfer rate in the heterojunction 3. Therefore, FEpTs with a large Dirac point shift are more sensitive to light than those with a small shift. The first or the basic minimum in the transfer curve represents the depletion regime properties of channel near bottom layer 12. The second minimum in the transfer curve of G-bottom FEpTs is attributed to the Dirac point of the middle graphene sheet, where a similar carrier transfer process occurs and where the control of the depletion regime by the gate voltage is also observed. However, the degree of control for the middle graphene sheet is obviously lower than for the bottom graphene channel due to the effect of the back-gate. The QD-bottom FEpTs show a small Dirac shift because the near bottom channel was mainly comprised of QDs instead of graphene. This results in a reduction in the transfer rate and the degree of gate control degree, which


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decrease even more for the middle and upper graphene sheet channels, resulting in an absence of transfer characteristics.

The mobility can calculated from the transfer characteristics according to the expression 10b, 13:

µ=

L ∂I ⋅ D WCoxVSD ∂VG

(1)

where W and L are the width and length of the channel, respectively. Cox is the capacitance as gate dielectric per unit area. With the parameters VSD = 0.2 V, W = 2.5 mm, L = 0.1 mm, and Cox ~ 100 pF, we determined the mobilities of the holes in the third quadrant and the electrons in the first quadrant to be µ H = 137 cm2 V−1 s−1 and 2 −1 −1 µ E = 147 cm V s , respectively, for the G-bottom FEpTs. Similarly, we also

determined µ H = 59 cm2 V−1 s−1 and µ E = 14 cm2 V−1 s−1 for the QD-bottom FEpTs. Therefore, the mobility of the G-bottom FEpTs was approximately an order of magnitude larger than that of the QD-bottom FEpTs for both holes and electrons. Photo responsivities



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Figure 5. Photo responsivities as a function of light irradiation of the QD-bottom FEpTs (a) and the G-bottom FEpTs (b). The physical mechanism of the sensing light is described as follows. Under light illumination, photo-induced excitons are generated in PbSe QDs and then separate into free electrons and holes. Due to the driving effect of a gate voltage, holes or electrons (depending on the polarity of gate voltage) are transferred from the QDs to graphene where carriers transport more effectively. Meanwhile electrons or holes in equal number remaining in the QDs form an electric field in the opposite direction to the gate electric field, which results in an additional photo-induced gate voltage termed as the “light-gate effect”. Due to this effect, horizontal gate voltage shifts (∆VG) can be used in FEpTs to calibrate for light irradiance (Ee) using the following expression 3: ∆VG = α Eeβ

(2)

where α and β are constants. According to equation (1), the increments of channel current (∆ISD) resulting from the light illumination can be expressed as a function of the gate shift (∆VG):

∆I SD =

W Cox µVSD∆VG L

(3)

For a specific device, WCox µ L is constant, and ∆ISD is directly proportional to VSD∆VG. However, ∆VG is difficult to measure directly. The responsivity (R) of a FEpT is always calculated from ∆ISD 6a:

R=

Iill − I Dark ∆I SD = P P


(4)

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where Iill and IDark are the channel current under light illumination and in dark conditions, respectively, and P = AEe is the incident optical power, where A is the illumination area. Substituting equation (3) into equation (4), one gets the following:  α CoxVSD µ lg(R) = lg  L2 

  + ( β − 1) lg( Ee ) 

(5)

Theoretically, lg(R) maintains a linear relationship with lg(Ee) according to equation (5). Therefore, a double-logarithmic system is used to plot R versus Ee. The responsivities of the QD-bottom and G-bottom FEpTs in the p-channel regime are shown in figure 5 (a) and (b) and VSD = −8 V and VG = −1 V and −4 V. A good linear dependence between lg(Ee) and lg(R) is observed. By fitting the data using equation (5), β was determined to be 0.02 and −0.1 for the QD-bottom and G-bottom FEpTs, respectively. According to equation (2), β represents the degree of photo-induced charge separation, and α corresponds to the degree of channel order. Because the channel design was the same for both FEpTs, α is the same for both devices. It is understood that the order of the QD layer and the graphene layer has a strong influence on β. Transfer is more effective when a stronger electric field is applied to a heterojunction. For back-gate FEpTs, a stronger electric field is applied to the bottom heterojunction more effectively in G-bottom FEpTs due to the closeness to the back-gate, which results in a high value of β. Photoresponse time of FEpTs



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Figure 6. The Transient photocurrent response of the QD-bottom FEpTs (a) and the G-bottom FEpTs (b) during several ON / OFF cycles at a light irradiation of 8.45 mW cm-2, a wavelength of 808 nm, VG = −5, VSD = −1 V and an on/off time of 50 s. Transient photo current response of the two FEpTs to On / Off illumination cycles were also measured, as shown in figure 6. The channel current decreased with illumination time and increased when the light was switched off. ID in G-bottom FEpTs during illumination was fit to an exponential equation: ∆I D = ∆I1 exp(−t / τ1 ) + ∆I 2 exp(−t / τ 2 )

with two rise times, τ1 and τ2. The time constants τ1 and τ2 are 12 s and 49 s, respectively. Similarly, when illumination is off, the channel current was fit to an exponential equation: ∆ I D = ∆ I 1 (1 − exp( − t / τ 3 )) + ∆ I 2 (1 − exp( − t / τ 4 ))

with two relaxation times, τ3 and τ4. The time constants τ3 and τ4 are 15 s and 63 s, respectively. As for the QD-bottom FEpTs, it was difficult to fit ID using the same exponential equations. The transient channel current response to light was reduced to exponential functions with one time constant. The rise time and relaxation time constants were determined to be 21 s and 56 s, respectively. 14 ACS Paragon Plus Environment

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The response time is related to the total amount of carriers transferred per unit time. The total amount is determined from net electrons or holes transferred to the graphene when either positive or negative gate voltage is applied. Therefore, the response time of the two devices showed large differences. For the G-bottom FEpTs, τ1 represents the transfer time of the photo-induced holes from QDs to graphene, and τ3 represents the lifetime of electrons in QDs before they transfer to neighboring graphene sheet(s) 3a

. Therefore, τ1 < τ3 indicates that QDs are negatively charged when exposed to light.

For the QD-bottom FEpTs, the transient ID does not have the double-time-constant feature, which results in the low transfer rate at the heterojunction where the weak back-gate electric field is applied. In the QD-bottom FEpTs, the shortest vertical distance, determined from SEM images (figure 2(b)), between the heterojunctions to the SiO2 film was approximately 30 ~ 36 nm, which was much larger than that of the G-bottom FEpTs (0.2 ~ 5 nm). Due to the structure of FEpTs, a higher distance induces a weaker back-gate electric field leading to a lower transfer rate. The carriers that transferred to and transported in graphene were drowned in the carriers transported directly in the QD layer. As a consequence, the rise time (or relaxation time) could not be divided into two rise (or relaxation) time constants.

4. CONCLUSIONS We investigated two types of MH-FEpTs that utilized graphene-PbSe QD hybrids. Through a comparative study, G-bottom FEpTs showed a multi-saturation behavior due to the multi-graphene layer effect, which was absent in the QD-bottom FEpTs. 15 ACS Paragon Plus Environment


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The mobilities of the electrons and holes were µ E = 147 cm2 V−1 s−1 and µ H = 137 cm2 V−1 s−1 in the G-bottom FEpTs and µ E = 14 cm2 V−1 s−1 and µ H = 59 cm2 V−1 s−1 in the QD-bottom FEpTs. Higher responsivity and faster response rate were both achieved by the G-bottom FEpTs. All these advantages in G-bottom FEpTs can be attributed to the back-gate effect. Therefore, higher performance is expected in those FEpTs where heterojunctions are designed to be close to the back-gate and are based on graphene and QD hybrids.

ASSOCIATED CONTENT AUTHOR INFORMATION

Corresponding Author Addressed correspondence to [email protected]

Present Addresses Institute of Laser & Opto-Electronics, College of Precision Instruments and Opto-electronics Engineering, Tianjin University, Tianjin 300072, China

Author Contributions The manuscript was written through contributions of all authors. All authors have approved the final version of the manuscript.

Funding Sources


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This work is supported by the National Natural Science Foundation of China (Grant No. 61271066) and the Foundation of Independent Innovation of Tianjin University (Grant No. 60302070).

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