Plasmonic Silicon Quantum Dots Enabled High-Sensitivity

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Plasmonic Silicon Quantum Dots Enabled High-Sensitivity UltraBroadband Photodetection of Graphene-Based Hybrid Phototransistors Zhenyi Ni, Lingling Ma, Sichao Du, Yang Xu, Meng Yuan, Hehai Fang, Zhen Wang, Mingsheng Xu, Dongsheng Li, Jianyi Yang, Weida Hu, Xiaodong Pi, and Deren Yang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b03569 • Publication Date (Web): 18 Sep 2017 Downloaded from http://pubs.acs.org on September 19, 2017

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ACS Nano

Plasmonic

Silicon

Ultra-Broadband

Quantum

Dots

Photodetection

of

Enabled

High-Sensitivity

Graphene-Based

Hybrid

Phototransistors

Zhenyi Ni, †,⊥ Lingling Ma, ‡,⊥ Sichao Du, ‡ Yang Xu,*, ‡ Meng Yuan,† Hehai Fang, § Zhen Wang, § Mingsheng Xu, ‡ Dongsheng Li, † Jianyi Yang, ‡ Weida Hu,*, § Xiaodong Pi, *, † & Deren Yang*, †

†

State Key Laboratory of Silicon Materials and School of Materials Science and Engineering,

Zhejiang University, Hangzhou, Zhejiang 310027, China. ‡

Key Laboratory of Micro-Nano Electronics and Smart System of Zhejiang Province, College of

Information Science and Electronic Engineering, Zhejiang University, Hangzhou 310027, China. §

State Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese

Academy of Sciences, Shanghai 200083, China

KEYWORDS: silicon quantum dots, boron doping, graphene, phototransistor, mid-infrared, localized surface plasmon resonance

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ABSTRACT

Highly sensitive photodetection even approaching the single-photon level is critical to many important applications. Graphene-based hybrid phototransistors are particularly promising for high-sensitivity photodetection because they have high photoconductive gain due to the high mobility of graphene. Given their remarkable optoelectronic properties and solution-based processing, colloidal quantum dots (QDs) have been preferentially used to fabricate graphene-based

hybrid

phototransistors.

However,

the

resulting

QD/graphene

hybrid

phototransistors face the challenge of extending the photodetection into the technologically important mid-infrared (MIR) region. Here, we demonstrate the highly sensitive MIR photodetection of QD/graphene hybrid phototransistors by using plasmonic silicon (Si) QDs doped with boron (B). The localized surface plasmon resonance (LSPR) of B-doped Si QDs enhances the MIR absorption of graphene. The electron transition based optical absorption of B-doped Si QDs in the ultraviolet (UV) to near-infrared (NIR) region additionally leads to photogating for graphene. The resulting UV-to-MIR ultra-broadband photodetection of our QD/graphene hybrid phototransistors features ultra-high responsivity (up to ∼ 109 A/W), gain (up to ∼ 1012) and specific detectivity (up to ∼ 1013 Jones).

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Graphene is emerging as a viable alternative to conventional optoelectronic materials1. Breakthroughs are highly expected in graphene-based optoelectronic devices that can be monolithically integrated into silicon (Si)-based mass-production platforms.2-4 Among all kinds of graphene-based optoelectronic devices phototransistors are attracting significant attention due to their highly sensitive photodetection and compatibility with Si-based read-out electronics.5-7 Since the intrinsic light absorption of graphene is rather weak,8-10 graphene is usually hybridized with other nanostructures to enhance the light absorption of graphene-based phototransistors.7, 11-18

Among these nanostructures colloidal quantum dots (CQDs) are particularly interesting

because they have excellent tunable optical absorption and suitability for low-cost solution-based processing.19-21 Up to now, CQD/graphene hybrid phototransistors have exhibited high-performance photodetection in the wavelength range from the ultraviolet (UV) to near-infrared (NIR) by taking advantage of photogating effect.5, 7, 13 With the growing demand for the photodetection in the mid-infrared (MIR) region it has been considered that narrow-bandgap CQDs such as colloidal HgTe QDs are promising to form hybrid phototransistors with graphene.22 However, the stability and toxicity of narrow-bandgap CQDs seriously challenge their practical use in a large scale.23, 24 As one type of the most important CQDs, Si QDs hold an advantageous position given the abundance and nontoxicity of Si, the stability of Si QDs and the compatibility of Si QDs with Si-based technologies.23, 25 It has been recently found that the doping of Si QDs with boron (B) and phosphorus (P) enables the localized surface plasmon resonance (LSPR) of Si QDs in the MIR region.26-29 Compared with P-doped Si QDs, B-doped Si QDs are currently better positioned for device applications in terms of dopant activation efficiency, oxidation resistance 3


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and dispersibility.26,

29, 30

This inspires the probable use of B-doped Si QDs as plasmonic

antennas to enhance the optical absorption of graphene in the mid-infrared region, analogous to classical plasmonic metal nanostructures.31-33 Given the fact that heavy-B-doping-induced band-tail states have already extended the optical absorption of Si QDs from the traditional UV/visible region into the NIR region,28 hybrid phototransistors based on graphene and Si QDs should exhibit the capability of UV-to-MIR ultra-broadband photodetection, which is in stark contrast with the narrowband photodetection of those based on graphene and plasmonic metal nanostructures. In this work, we use colloidal Si QDs heavily doped with B to form hybrid phototransistors with graphene. The LSPR of B-doped Si QDs significantly enhances the MIR absorption of graphene, enabling the MIR photodetection of QD/graphene hybrid phototransistors. In addition, the photogating effect following the band-to-band and subbandgap optical absorption of B-doped Si QDs causes QD/graphene hybrid phototransistors to effectively operate in the UV-to-NIR region. Therefore, ultra-broadband UV-to-MIR photodetection of QD/graphene hybrid phototransistors with high sensitivity is realized. The current work advances the integration of graphene with Si technologies for high-performance optoelectronic devices.

RESULTS AND DISCUSSION Si-QD/graphene phototransistor structure. Figure 1a schematically shows the structure of a hybrid phototransistor based on graphene and Si QDs. Graphene chemically vapor-deposited on Cu foil is transferred to the 10 µm long channel region between two Cr/Au

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Figure 1. Structure of a Si-QD/graphene phototransistor. (a) Schematic diagram of the structure of the hybrid phototransistor based on B-doped Si QDs and graphene. (b) Two distinct optical phenomena of B-doped Si QDs may be exploited during the phototransistor operation. In the MIR region, the intense near field associated with the LSPR of B-doped Si QDs may enhance the direct excitation of graphene, enabling the device to efficiently respond to the MIR light. The UV-to-NIR absorption of Si QDs leads to photogenerated carriers (electrons and holes) in the QDs. Transfer of one type of carriers from the QDs to graphene and trapping of the other type of carriers in the QDs (i.e., photogating) then take place, rendering the photoresponse of the device.

electrodes on the top of a SiO2/Si substrate. Colloidal B-doped Si QDs are spin-coated onto graphene to form a complete device structure. The prepared device is a phototransistor with the backgate of the heavily-doped p-type Si substrate and the channel of Si-QD/graphene. Figure 1b illustrates that two distinct optical phenomena of Si QDs may be exploited during the phototransistor operation. The LSPR of Si QDs induces an intense electric field, which enhances the direct excitation of the underlying graphene in the MIR region. Hence, the device is able to efficiently respond to the MIR light. The UV-to-NIR absorption of Si QDs leads to photogenerated carriers (electrons and holes) in the QDs. Transfer of one type of carriers from the QDs to graphene and trapping of the other type of carriers in the QDs (i.e., photogating) then take place, rendering the photoresponse of the device. 5


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Si QDs heavily doped with B used in the phototransistors are synthesized by using SiH4-based nonthermal plasma.27, 28, 30 The nominal B concentration of Si QDs is 40%, which has been measured by using inductively coupled plasma atomic emssion spectroscopy. Colloidal Si QDs can be readily formed by dispersing Si QDs in ethanol, as shown in Figure 2a. The low-resolution transimission electron microscopy (TEM) image (Figure 2b) demonstrates that nearly spherical Si QDs are syntheiszed in this work. The crystallinity of Si QDs is evidenced by the selected-area electron diffraction (the inset of Figure 2b) and the high-resolution TEM image of an individual Si QD (Figure 2c). A statistical size analysis indicates that Si QDs have a mean size of ~ 6 nm (Figure 2d). Figure 2e shows the MIR absorption of Si QDs. It is seen that a LSPR peak occurs at ~ 3 µm, which can be well fitted by the Mie absorption theory including the Drude contribution (Supporting Information).26, 28, 34 The overtone at ~ 4 µm is related to the vibration of boron hydride at the surface of Si QDs.27 From the fitting we work out that the hole concentration of Si QDs is ∼ 4.5 ×1020 cm-3. This indicates that only ∼ 2.2% of doped B atoms are effectively activated to produce free holes, consistent with previously observed partial electrical activation of B in Si QDs.26, 35 It is the collective oscillation of free holes in Si QDs that gives rise to the LSPR of Si QDs in the MIR region.26-28 The UV-to-NIR absorption spectrum of colloidal Si QDs is shown in Figure 2f. In addition to the conventional band-to-band transistion based optical absorption in the UV/visible region (Figure S1a in the Supporting Information), the heavy B doping extends the optical absorption of Si QDs into the NIR region because the heavy B doping introduces band-tail states to Si QDs, giving rise to subbandgap transistion.28 By fitting the UV-to-NIR absoprtion onset of Si QDs we find that the width of the band-tail states is ∼ 0.74 eV (Figure 6


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Figure 2. Characterization of Si QDs and Si-QD/graphene. (a) A photograph of an ethanol solution of B-doped Si QDs with the concentration of 0.025 mg/ml. (b) Low-resolution TEM image of B-doped Si QDs. The selected area electron diffraction is shown in the inset. (c) High-resolution TEM image of a B-doped Si QD. 7


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(d) Size distribution with a log-normal fit for B-doped Si QDs. The mean size of B-doped Si QDs is ~ 6 nm. (e) FTIR spectrum (black line) of B-doped Si QDs. The LSPR-induced absorption peak is fitted with the Drude model (red line). (f) Optical absorption spectrum of B-doped Si QDs (red line) and absorption coefficient (α) of a B-doped Si-QD film. (g) Raman spectra of graphene and Si-QD/graphene.

S1b in the Supporting Information). When colloidal Si QDs are spin-coated to form a film, the UV-to-NIR absorption of the film is also significantly strong. We have worked out the absorption coefficient of the Si-QD film, as shown in Figure 2f. It is significant that the absorption coefficient of the Si-QD film is much larger than that of bulk Si in nearly the whole UV-to-NIR region.36 Figure 2g shows the Raman spectra of graphene and Si-QD/graphene. The ratio of the intensity of the 2D peak at ∼ 2692 cm -1 to that of the G peak at ∼ 1591 cm-1 for graphene is larger than two, implying that monolayer graphene is used in the current work.37 The absence of the D peak at ∼ 1350 cm-1 for graphene means that negligible defects occur to our graphene.38 The Si-B related peak at ∼ 2434 cm-1 (Figure S2 in the Supporting Information) together with the aforementioned 2D and G peaks in the Raman spectrum indicates that the hybrid structure of Si QDs and graphene is indeed formed in the channel region of a phototransistor. The thickness of the Si-QD film is ~ 105 nm (Figure S3a in the Supporting Information), which has been found to render optimum overall device performance. (Figure S3b in the Supporting Information).

Ultra-broadband photoresponse. Figure 3a shows the drain-source photocurrent density (JPh) as a function of the drain-source bias (VDS) for a Si-QD/graphene phototransistor under the UV-to-MIR illumination at the gate voltage (VG) of 0 V. The UV-to-NIR and MIR 8


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Figure 3. Ultra-broadband UV-to-MIR photoresponse. (a) Photocurrent density (JPh) as a function of VDS at VG = 0 V from the UV to MIR. The irradiances of the MIR illumination and the UV-to-NIR illumination are 375 mW/cm2 and 0.2 µW/cm2, respectively. Please note that the JPh is plotted in a semi-logarithmic scale. (b) Responsivity as a function of laser irradiance at different laser wavelengths at VG = 0 V and VDS = 1 V. (c) Gain 9


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as a function of illumination wavelength at VG = 0 V and VDS = 1 V. The illumination irradiances in the MIR and UV-to-NIR regions are 375 mW/cm2 and 0.2 µW/cm2, respectively. (d) Time-dependent photocurrent density of the Si-QD/graphene photodetector at different illumination wavelengths at VG= 0 V and VDS=1 V. (e) Gain as a function of the inverse of carrier transit time (ttr−1). The wavelength and irradiance of the illumination are 532 nm and 0.2 µW/cm2, respectively. (f) Spectral dependence of the NEP and specific detectivity (D*) of the device. The UV-to-NIR and MIR measurements were conducted at room temperature and 77 K, respectively.

measurements were conducted at room temperature and 77 K, respectively. It is found that JPh linearly increases as VDS increases from 0 to 1 V (please note that JPh is in a logarithmic scale in Figure 3a). When the phototransistor is illuminated at the irradiance of 375 mW/cm2 in the MIR region (2.5 – 3.9 µm), JPh increases with the increase of the incident light wavelength (λ) from 2.5 to 3.0 µm and then decreases as λ further increases from 3.0 to 3.9 µm. For the illumination at the irradiance of 0.2 µW/cm2 in the UV to NIR region (λ = 375 – 1870 nm), the maximum JPh is obtained as λ is ∼ 532 nm. The responsivity (R) of the phototransistor as a function of the illumination irradiance at VDS = 1 and VG = 0 V is illustrated in Figure 3b. The decrease of R with the increase of the irradiance is typical for photodetectors.7, 14, 17 R is in the range of 0.22 – 44.9 A/W for the MIR illumination at the lowest irradiance of 375 mW/cm2, while R changes between 1.2 × 108 and 2.2 × 109 A/W for the UV-to-NIR illumination at the lowest irradiance of 0.2 µW/cm2. It should be pointed out that a phototransistor with only graphene in the channel hardly responds in the MIR region. We find that Si QDs may enhance R by the factor of ∼ 1011 in the UV-to-NIR region by comparing the Si-QD/graphene phototransistor with the graphene-only phototransistor (Figure S4 in the Supporting Information).

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Figure 3c shows the dependence of the photoconductive gain of the Si-QD/graphene phototransistor on λ at VDS = 1 and VG = 0 V. Ultra-high gains in the range from ~ 7×1011 to 2.6×1012 have been obtained for the photodetection from the UV to MIR. The gain is calculated by using τ/ttr (τ is the time for the carrier trapping and ttr is the time for the transfer of carriers through the device channel).39 Since we know ttr = L2/(µVDS) (L is the length of the device channel and µ is the carrier mobility), we can work out that ttr is ~ 4 ps. τ may be approximated by the fall time of the transient Jph during the ON/OFF illumination, as shown in Figure 3d. It is found that for the UV to MIR illumination τ is in the range of ∼ 3.4 – 9.0 s, which is comparable to those for the phototransistors fabricated by Roy et al.6 and Lopez-Sanchez et al..40 Please note that τ for the Si-QD/graphene photodetector is similar to that for the photodetector only with Si QDs in the channel (Figure S5 in the Supporting Information). This implies that the carrier trapping is mainly associated with Si QDs, which may have trap states induced by defects (e. g., dangling bonds) at the QD surface.41 It is the large difference between τ and ttr that leads to the ultra-high gain in this work. Figure 3e shows the change of the gain with the inverse of ttr under the illumination at λ = 532 nm with the irradiance of 0.2 µW/cm2. As ttr decreases from 256 to 4 ps, the gain increases from ~ 3 × 1010 to 2 × 1012. The linear dependence of the gain on the inverse of ttr is consistent with the photoconductive structure of our devices.42 We should be able to further increase the gain by reducing the Si-QD/graphene channel length because ttr decreases with the decrease of L. As shown in Figure S6 in the supporting information, we have also calculated the external quantum efficiency (EQE) and internal quantum efficiency (IQE) of the device by using EQE = Ip/q · hν/P and IQE = Ip/q · hν/αP, respectively (where Ip is the photocurrent of the device, q is the elemental charge, h is the 11


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Planck constant, v is the frequency of light, P is the power of light and α is the absorbance of the Si-QD/graphene structure).36 The EQE of the device varies from ~ 0.07 to 18 in the MIR region, while it changes from ~ 108 to 5×109 in the UV-to-NIR region. The IQE of the device ranges from ~ 7 to 539 and from ~ 109 to 6×1010 in the MIR and UV-to-NIR regions, respectively. The figure of merit used to evaluate the capability of weak light detection for a photodetector is the noise equivalent power (NEP), which can be evaluated by considering the 1/f noise, shot noise and thermal noise of the device.43, 44 The calculation of the spectral density of these noises are detailed in the Supporting Information. At the modulation frequency of 1 Hz, the values of spectral density of 1/f noise (SI (1/f)), shot noise (SI (shot)) and thermal noise (SI (thermal) are ~ 1.70×10-19, 8.91×10-22 and 4.41×10-23 A2/Hz, respectively (Figure S7 in the Supporting Information). With the measured responsivity (R) of the device at the modulation frequency of 1 Hz at VG = 0 V and VDS = 1 V, the NEP can be calculated by using NEP = (SI = SI (1/f) +SI (shot) + SI (thermal)).39 The obtained values of NEP are as low as ~ 10-10 and 10-18 W/Hz0.5 in the MIR region and the UV-to-NIR region, respectively (Figure 3f). The rather small values of NEP indicate that the Si-QD/graphene photodetectors may be well used for weak light detection. By using

(S is the area of the device), we have calculated the

specific detectivity of D*, which is also shown in Figure 3f. The values of D* are in the orders of 105 and 1013 Jones for the MIR photodetection and the UV-to-NIR photodetection, respectively. This signifies the excellent photodetection sensitivity of the Si-QD/graphene phototransistor.

LSPR and photogating. To elucidate the mechanism for the photoresponse of Si-QD/graphene phototransistors in the MIR region, we have carried out finite-difference 12


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time-domain (FDTD) simulation for the distribution of square electric field at Si QDs and the underlying graphene. Figure 4a representatively shows the results for the illumination at λ = 2.7, 3.0 and 3.6 µm. It is clear that the LSPR of Si QDs introduces intense electromagnetic fields in their proximity. The electromagnetic field beneath a Si QD is the most significantly enhanced when the illumination is at λ = 3.0 µm. The enhanced electromagnetic fields would significantly increase the optical absorption of graphene, resulting in a prominent photoresponse.45, 46 The highest responsivity obtained at λ = 3.0 µm (Figure 3b) is consistent with the most intense

Figure 4. Localized surface plasmon resonance, photogating and carrier transfer. (a) Cross-section distribution of the square of electric field (|E|2) at B-doped Si QDs and graphene under the illumination at the wavelength of 2.7, 3.0 and 3.6 µm. (b) Drain-source current density (JDS) as a function of VG at 3 µm and 532 nm with irradiances of 375 mW/cm2 and 0.2 µW/cm2, respectively. The MIR measurements were conducted at 77 K. Those for Si-QD/graphene and graphene-only devices in the dark are also shown. (c) Schematic diagram of the band structures of graphene and Si QDs and the tendency of charge transfer between them. (d) Schematic diagram of the band structure of Si-QD/graphene and charge transfer after the UV-to-NIR illumination.

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LSPR of Si QDs at λ = 3.0 µm (Figure 2e). We have measured the dependence of the drain-source current density (JDS) on VG before and after the MIR illumination. It is seen that the Dirac point (VD) of graphene hardly changes with the MIR illumination, as representatively shown in Figure 4b for the illumination at λ = 3.0 µm. This means that the photoresponse in the MIR region is merely induced by the LSPR enhanced optical absorption of graphene. There is no transfer of photogenerated carriers from Si QDs to graphene. We have found that a phototransistor with the structure of undoped Si-QD/graphene in the channel has negligible photoresponse in the MIR region, similar to the graphene-only phototransistor (Figure S4 in the Supporting Information). This is consistent with the fact that the LSPR does not occur to undoped Si QDs.26 Since it is well known that the LSPR of Si QDs depends on their doping level,26 one may expect that the photoresponse of the Si-QD/graphene phototransistor in the MIR region should be readily tuned by the doping level of Si QDs. We have demonstrated such tuning by using Si QDs with a lower B doping level (the hole concentration of ∼ 3.3 ×1020 cm-3). With the decrease of the B doping level, the peak and intensity of the LSPR of Si QDs redshifts and decreases, respectively (Figure S8 in the Supporting Information). Hence, for the Si-QD/graphene phototransistor with the lower B doping level the wavelength at which the maximum photoresponse appears increases, while the photoresponse of the device is attenuated (Figures S4 and S9 in the Supporting Information). It should be noted that the LSPR of Si QDs is also dependent on the QD size.28,

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The distribution density of Si QDs determines the spacing

between Si QDs and thus the coupling of the LSPR of neighboring Si QDs.26 Our FDTD simulations have verified that both the size and distribution density of Si QDs indeed affect the electromagnetic field in graphene via the LSPR (Figures S10 and S11 in the Supporting 14


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Information). Therefore, it can be envisioned that the performance of the Si-QD/graphene phototransistor may be further improved by enhancing the LSPR-induced absorption of graphene through the systematic optimization of the doping level, size and distribution density of Si QDs. Figure 4b shows that the device with only graphene in the channel has a Dirac point (VD) at ~ 39 V. After Si QDs are placed atop graphene, VD is shifted to ∼ 9 V. This implies the electron transfer from Si QDs to graphene, as schematically shown in Figure 4c. The Si QDs induced redshift of the 2D and G peaks of graphene in the Raman spectra (Figure 2g) actually also indicates this electron transfer.47 Moreover, our ultraviolet photoelectron spectroscopy (UPS) study shows that the work functions of Si QDs and graphene are - 4.62 and -4.78 eV, respectively, further validating the electron transfer from Si QDs to graphene. After the Fermi level of Si QDs becomes the same as that of graphene, the valence band maximum (VBM) and conduction band minimum (CBM) of Si QDs bend upward at the Si-QD/graphene interface (Figure 4d). When band-to-band or subbandgap electron transition occurs in Si QDs under the UV-to-NIR illumination, the resulting photogenerated holes in Si QDs readily transfer to graphene because of the upward bending of the energy bands at the Si-QD/graphene interface (Figure 4d). Therefore, the transfer of photogenerated holes from Si QDs to graphene makes VD shift to a more positive value, as representatively shown in Figure 4b for the illumination at

λ = 532 nm. It is seen that the highest device responsivity is obtained at λ = 532 nm in the UV to NIR region (Figure 3a-b). This indicates that the absorption depth of the Si-QD film at λ = 532 nm is rather comparable with the current Si-QD film thickness (∼ 105 nm). As λ decreases toward 375 nm, the optical absorption of the Si-QD film mainly occurs in the near-surface region of the Si-QD film. The photogenerated holes may not efficiently transfer to the 15


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underlying graphene. As λ increases toward 1870 nm, however, the optical absorption in the Si-QD film is reduced because of the increased absorption depth. From the dependence of Jph on VG for the illumination at λ = 532 nm (Figure 4b) we can infer that R in the UV-to-NIR region may be also modulated by VG. This modulation is based on the VG-induced change of the Fermi level of graphene. For example, R can increase to ∼ 3×109 A/W as VG becomes - 1 V (VG - VD = - 16 V) for the illumination at λ = 532 nm (Figure S12 in the Supporting Information). In fact, the optical absorption of Si QDs in the UV-to-NIR region also depends on the doping level.28 For a lower B doping level (the hole concentration of ∼ 3.3 ×1020 cm-3) we see the attenuated photoresponse in the UV-to-NIR region (Figure S4 in the Supporting Information), consistent with the weakened Si-QD absorbance (Figures S1a in the Supporting Information).

Ultra-high responsivity and sensitivity. QDs and graphene are key player among materials used to fabricate high-performance photodetectors.5, 39, 48-54 We have compiled R and D* for QD- and graphene-based photoconductive detectors in Figure 5. It is clear that the values of R in the order of ∼ 10 A/W for the MIR region and those in the order of ∼ 109 A/W for the UV-to-NIR region obtained in the current work are all higher than previous ones. The present values of D* in the order of ∼ 1013 Jones for the UV-to-NIR region are at the top level compared with those reported before. Although the values of D* seriously decrease to be in the order of ∼ 105 Jones for the MIR region, they actually meet the photodetection requirement.36 We would like to point out that the Si-QD/graphene phototransistors now have relatively slow photoresponse dynamics, which should work well in a series of technologically important areas such as 16


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bioimaging,55 medical sensing56 and low-light photography40. It can be envisioned that both backgate pulses40, 57 and topgate bias11 may be employed to sweep out the trapped carriers of Si QDs within an appropriate time frame in the future. This would decrease the device response time down to the order of microsecond, broadening the use of the Si-QD/graphene phototransistors toward high-speed photodetection.

Figure 5. Comparison of the current results with those reported in literature. (a) Responsivity and (b) specific detectivity obtained in the current work are compared with those of related devices reported in literature.

CONCLUSION In summary, the doping-induced tunability of the optical absorption of Si QDs has been employed to vastly expand the photodetection range of QD/graphene hybrid phototransistors. The electron transition based subbandgap optical absorption of B-doped Si QDs extends the photodetection of QD/graphene hybrid phototransistors from the usual UV/visible region into the NIR region, while the LSPR of B-doped Si QDs further pushes the photodetection of QD/graphene

hybrid

phototransistors

into

the

MIR

region.

Si-QD/graphene

hybrid

phototransistors demonstrated in this work exhibit ultra-high responsivity, gain and specific detectivity throughout the ultra-broad UV-to-MIR range. The current work has important 17


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implication for the development of high-performance optoelectronic devices based on the hybrid structures of zero-dimensional plasmonic QDs (e. g., copper chalcogenide QDs)58, two-dimensional materials such as graphene.

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METHODS Si QD preparation. B-doped Si QDs were synthesized by means of non-thermal plasma. A mixture of SiH4/Ar (20%/80% in volume), B2H6/Ar (0.5%/99.5% in volume) and Ar was introduced into a 10 mm quartz tube which was used as the plasma chamber. The flow rates of SiH4/Ar, B2H6/Ar and Ar were set at 12, 157 and 105 sccm (standard cubic centimeter per minute), respectively. The pressure of the chamber was maintained at ~ 2 Torr during the synthesis of the QDs. Plasma was generated with a 13.56 MHz power source and a matching network. The power coupled into the plasma was ~ 105 W. B-doped Si QDs colloid was prepared by adding as-synthesized B-doped Si QDs into anhydrous ethanol with ultra-sonication. As a homogeneous distribution of the QDs and a precise control of the film thickness are curial to the performance of photoconductive detector, a spin coating method was used to control the thickness of Si-QD film on graphene. Multi-coating and solvent evaporation were employed to obtain the appropriate thickness of Si-QD film. Si QD Characterization. Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) measurements were performed by using Thermal Scientific iCAP6300. B-doped Si QDs were dissolved in a 2 mol L-1 NaOH solution then diluted with ultrapure water to make the concentration of the QDs be ~10 µg/ml (10 ppm). HCl was used to neutralize the solution before the measurement. TEM measurements were performed by using JEM 2100F at an acceleration voltage of 200 kV. Samples were prepared by drop-casting diluted solutions of B-doped Si QDs on copper grids coated with ultra-thin carbon film. A UV-to-NIR spectrometer (HITACHI U4100) was employed to measure both diluted Si-QDs colloid contained in a quartz cuvette with a path length of 10 mm and Si-QD films spun-coated on a quartz glass substrate at 19


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the resolution of 4 nm. FTIR measurements were carried out by using BRUKER VERTEX 80v with a resolution of 4 cm-1. Si QDs were drop-casted onto a potassium bromide (KBr) substrate for the FTIR measurements in the absorption mode. Each FTIR spectrum was collected after the solvent totally evaporated. Ultraviolet photoelectron spectroscopy (UPS) measurements were performed by using Thermo Scientific ESCALAB 250Xi with a 21.2 eV He-Ia source and a -9.8 eV bias. Samples were prepared by spin-coating Si QDs on ITO-covered glass substrates. SEM images were obtained by using a Hitachi S4800 field emission microscope at an acceleration voltage of 5 kV. Raman spectroscopy was measured by using a confocal Raman microscope (Senterra, BRUKER) and a Renishaw® Invia system. The excitation was carried out with a 532 nm laser. Photodetector Fabrication. Monolayer graphene was grown by CVD using our previous standard process. Before transfer, we etched the graphene that was grown on the backside of Cu foil with O2 plasma to avoid the residue of Cu on graphene. After etching, graphene/Cu was spun-coated (with 500 rpm for 5 s and 4000 rpm for 60 s) with a PMMA (< 100 µm) layer. Then the PMMA/graphene/Cu sample was put into the Cu etching solution (5H2O·CuSO4: HCl: H2O = 15.6 g: 50 ml : 50 ml) for more than 12 hours to remove the Cu. The PMMA/graphene film was rinsed in deionized water for 5 mins and transferred onto a SiO2 substrate. The PMMA was washed off using dichloromethane (DCM) at 50 °C and isopropyl alcohol (IPA). Devices were fabricated on a commercially purchased highly p-doped Si wafer (the resistivity < 0.01 Ω•cm) with a 300 nm SiO2 layer. Monolayer graphene was patterned by photolithography and etched by O2 plasma to form a 3 µm × 10 µm ribbon. Si QDs (2 mg/mL in anhydrous ethanol) were spun-coated onto the pre-patterned graphene with at a speed of 2000 rpm. All the devices were 20


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annealed at 300°C under H2 atmosphere for 1 hour before the measurements to remove the residues and contaminants. Photodetector characterization. The photocurrent was measured by using Agilent Semiconductor Analyzer B1500. The photocurrent density is calculated by dividing the measured photocurrent by the width of the graphene channel. In the UV to NIR region, nine lasers with wavelengths of 375, 405, 532, 780, 808, 980, 1342, 1450 and 1870 nm were used. Lasers with wavelengths of 2.5, 2.7, 3.0, 3.3, 3.6 and 3.9 µm were used in the MIR region. The MIR measurements were conducted in the vacuum with a pressure lower than 10-4 Torr at 77 K. For the measurement of the transient response of the device, a continuous-wave laser is employed during the measurement. The on and off time of the laser is controlled by a mechanical shutter. FDTD Simulation. The interaction between Si QDs and graphene illuminated under normal incident plane light source was simulated by a FDTD software from Lumerical Solutions Inc. Si QDs with diameters of 6 nm were placed on a graphene sheet. The perfectly matched layer (PML) and periodic boundary conditions were utilized in source incident direction and perpendicular detection, respectively. The distribution of the near-field electric field intensity was obtained though established monitor.

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ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX. Analysis of additional experimental results and characterization data. (Figures S1-S12). (PDF)

AUTHOR INFORMATION Corresponding Authors *

E-mail: [email protected].

*

E-mail: [email protected]

*


*


Author Contributions ⊥

These authors contributed equally to this work. X.D.P. and Y.X. conceived the project. Z.Y.N.

and L.L.M. prepared the materials and fabricated the devices. Z.Y.N. and L.L.M. characterized the materials. Z.Y.N., L.L.M. S.C.D., H.H.F., Z.W. measured the devices. M.Y. and D.S.L. performed the FDTD Simulation. M.S.X and J.Y.Y. participated in the data analysis. X.D.P., Y.X., W.D.H and D.Y. supervised the project. X.D.P., Z.Y.N., L.L.M., S.C.D. and Y.X. wrote the manuscript, which has been reviewed by all the authors.

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is mainly supported by the National Key Research and Development Program of China (Grant No. 2017YFA0205700) and Natural Science Foundation of China (NSFC) (Grant Nos. 61774133, 61674127, 11734016 and 61474099). Partial support from the Zhejiang Natural Science Foundation (ZJ-NSF) (Grant No. LZ17F040001), the National Key Research and Development Program of China (Grant No. 2016YFA0200204) and the Fundamental Research 22


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Funds for the Central Universities (Grant No. 2016XZZX001-05) is acknowledged. The authors are grateful to the support of the micro-/nano-fabrication platform of Zhejiang University, the Cyrus Tang Center for Sensor Materials and Applications and the Fellowship of Churchill College at University of Cambridge.

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