Article Cite This: ACS Photonics XXXX, XXX, XXX−XXX
pubs.acs.org/journal/apchd5
P‑Type Doping of WS2 Quantum Dots via Pulsed Laser Ablation Septem P. Caigas,† Min-Chiang Cheng,‡ Tzu-Neng Lin,† Svette Reina Merden S. Santiago,† Chi-Tsu Yuan,† Chun-Chuen Yang,† Wu-Ching Chou,‡ and Ji-Lin Shen*,† †
Department of Physics and Center for Nanotechnology, Chung Yuan Christian University, Chung-Li 32023, Taiwan Department of Electrophysics, National Chiao Tung University, Hsinchu 1001, Taiwan
‡
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S Supporting Information *
ABSTRACT: Doping provides an advantage to engineering the optical and electrical characteristics of transition-metal dichalcogenides (TMDs). Here, we report the doping of WS2 quantum dots (QDs) with diethylenetriamine (DETA) using pulsed laser ablation. The synthesized DETA-doped 2H-WS2 QDs with an average size of ∼6 nm have been demonstrated by transmission electron microscopy. With the introduction of DETA during pulsed laser ablation, current modulation, carrier concentration, and field-effect mobility are greatly enhanced, demonstrating a successful doping in WS2 QDs. The positive shift of the threshold voltage in gatedependent conductance measurements reveals p-type doping for DETA-doped WS2 QDs. A remarkable improvement in photoluminescence in WS2 QDs by 74-fold has been achieved after DETA doping. An anomalous dopant-dependent negative photoconductivity was observed for WS2 QDs, originating from light-induced desorbing of water (oxygen) molecules on the surface. The proposed doping approach can provide a vehicle to modulate the optical and electrical properties in WS2 QDs and could be important in the performance improvement of WS2-QD-based devices. KEYWORDS: WS2 quantum dot, doping, photoluminescence, conductivity, pulsed laser ablation
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carriers’ radiative recombination rate, which is advantageous for potential applications in bioimaging and optoelectronic devices. The doping of TMD monolayers has been implemented via different strategies such as ultrasonic dispersion, vacuum deposition, and chemical doping.9,12−16 The modulation of the optoelectronic properties in TMD monolayers by tuning the doping concentration through chemical doping has been investigated.17−19 However, to our knowledge, there is no report describing the preparation or characterizations of doping for TMD QDs. Doping in TMD QDs not only provides a vehicle to understand fundamental properties, but it is also essential for potential applications using TMD QDs.6,20−22 For example, the conductivity in TMD QDs is attributed to the transport of photogenerated charged carriers, described by hopping or tunneling through the interparticle medium. Control of doping in TMD QDs can be an effective method to increase the carrier’s concentrations in TMD QDs and improve their transport properties. In addition, luminescence in TMD materials is associated with the interplay between excitons and trions.18 Pristine TMDs, such as WS2 or MoS2, reveal n-type semiconductors’ intrinsic characteristics, which is dominated by trions (i.e., charged excitons).23 A suitable p-type doping can counteract electron
he popularity of graphene and other two-dimensional materials has paved the way for recent discoveries for transition metal dichalcogenides (TMDs). Graphene is a representative two-dimensional material and has exceptional optical, thermal, magnetic, and mechanical properties.1 However, because of the lack of a bandgap, graphene has limited applications in the electronic and optoelectronic devices. TMDs are composed of a transition metal sandwiched between two chalcogens and exhibit a wide range of energy bandgaps, which makes them very useful in device applications. When the lateral dimension of TMDs is in nanoscale, it is transformed into quantum dot (QD) derivatives. TMD QDs have been prepared through a variety of techniques that involves either long hours of fabrication process through sonication2−4 or the aid of organic solvents.2,5 TMD QDs exhibits high tunability of the energy band gap due to quantum confinement effect.6 TMD QDs can offer large surface-tovolume ratio and good amenability to decorate with other nanomaterials, which is advantageous for potential applications in photocatalysis, sensors, optoelectronics, and bioimaging.4,7,8 Doping heteroatoms into TMDs or hybridizing TMDs with metal nanoparticles can modulate their electronic and optical characteristics, leading to many interesting phenomena in TMDs such as enhanced excitonic transitions, many-body interactions, phase transition, and plasmon-exciton coupling.9−13 Especially, doping of TMDs may increase their © XXXX American Chemical Society
Received: July 11, 2018
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DOI: 10.1021/acsphotonics.8b00941 ACS Photonics XXXX, XXX, XXX−XXX
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to the concentration of the black WS2 solution (including nanosheets, nanoparticles, and QDs).24 Supporting Figure S2 shows the dependence of the laser wavelength on the production yield of WS2 QDs by fixing other laser parameters including pulse energy, repetition frequency, and focus area. Compared with other laser wavelengths, the wavelength at 415 nm produces the highest yield (46.2%) for generating WS2 QDs. Thus, this wavelength was set for the subsequent synthesis of the DETA-doped WS2 QDs. The TEM and HRTEM images of the pristine and DETAdoped WS2 QDs are shown in panels a and b of Figure 2,
concentrations, leading to enhanced neutral excitons and inhibit the formation of trions, advantageous for radiative recombination. Thus, p-type doping in TMD QDs could be a feasible way to modify the carrier type and improve luminescence efficiency. Pulse laser ablation (PLA) is a top-down approach to synthesize nanomaterials under highly nonequilibrium conditions with local high temperatures and high pressures.23 Synthesis of TMD QDs using PLA is a one-step, time-saving method, which can be carried out in an ambient atmosphere. Doping of TMD QDs is expected to be realized by PLA because the dopant can be introduced into the solvent as the precursor for ablation by laser pulses. In this paper, we demonstrate a controllable doping of WS2 QDs with WS2 ultrafine powders as the WS2-QD source and diethylenetriamine (DETA) as the dopant (Figure 1a,b). The structural,
Figure 1. Schematic presentation of (a) experimental setup for pulse laser ablation (PLA) and (b) preparation process of the WS2 quantum dots (QDs) doped with diethylenetriamine (DETA).
electrical, and optical properties of the DETA-doped WS2 QDs were investigated using transmission electron microscopy (TEM), atomic force microscopy (AFM), Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), gate-dependent conductance measurement, photo current−voltage (I−V) characteristics, and photoluminescence (PL). The results from the electrical transport indicate that the introduction of WS2 QDs using DETA produces a p-type doping. The carrier concentrations, mobility, and PL of WS2 QDs can be controlled by varying the DETA concentration. DETA-doped WS2 QDs exhibit an evident negative photoconductivity (NPC) effect (i.e., the conductivity is suppressed under light emission). The NPC effect depends on the intensity of the incident light, environmental atmosphere, relative humidity (RH), and DETA concentrations. A mechanism that gives rise to NPC is proposed.
Figure 2. TEM images of (a) pristine and (b) DETA-doped WS2 QDs. HRTEM images of (c) pristine and (d) DETA-doped WS2 QDs. Intensity profiles along the lines marked in images (c) and (d) are shown in images (e) and (f). Insets of (a,b): size distribution of pristine and DETA-doped WS2 QDs. Insets of (c,d): FFT patterns of pristine and DETA-doped WS2 QDs.
respectively. The QDs were monodispersed in the range of 4− 7 nm, with the average size of ∼5.5 ± 1.0 nm and ∼5.8 ± 1.0 nm for the pristine and DETA-doped WS2 QDs, respectively (insets in Figure 2a,b). Figure 2c,d displays the HRTEM images from the marked rectangular area in panels a and b of Figure 2, respectively. The images have uniform interlayer spacing with ordered lattice fringes, revealing that both of the WS2 QDs were well crystallized. The insets in Figure 2c,d shows the fast Fourier transform (FFT) patterns of the pristine and DETA-doped WS2 QDs, respectively. The diffraction spots reveal distortions of the hexagonal structure, which are due to a tilt in the normal surface of QDs to the electron beam. The diffraction rings around the center of the circle were due to the amorphous carbon on the copper grid. By analyzing the intensity profile in HRTEM, it is possible to distinguish whether a WS2 lattice is a 2H phase or a 1T phase.25,26 Figure 2e,f displays the intensity profiles of the marked line in Figure 2c,d, respectively. The intensities from the S sites are comparable to those from the W sites in both samples. This minor intensity contrast between S and W sites indicates the
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RESULTS AND DISCUSSION The WS2 suspension displayed a dark-gray color after PLA, and some precipitates were aggregated on the top and bottom surfaces (Supporting Figure S1a). After centrifugation, the suspension turned light yellow (Supporting Figure S1b), and WS2 QDs were extracted from the transparent supernatant. To find the effects of the laser wavelength on the production yield of WS2 QDs, the laser wavelength in PLA was adjusted to the range of 415−490 nm. The production yield is defined as the concentration of the light-yellow supernatant solution relative B
DOI: 10.1021/acsphotonics.8b00941 ACS Photonics XXXX, XXX, XXX−XXX
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related to the bending vibration of water.34 For the DETA doped WS2 QDs, five extra peaks appeared at around 965, 1160, 1320, 1460, and 1580 cm−1 (marked by dotted lines), which are associated with DETA and assigned as the N−H wag vibration, the C−N stretching vibration, and the N−H bending vibration.35−37 In Figure 4b, all the absorption bands appear in DETA can be observed in the spectra of the DETA doped WS2 QDs. This indicates that DETA was likely grafted to WS2 QDs in the DETA-doped WS2 QDs, but it was not decomposed during PLA process. Figure 5a shows the XPS survey spectra for DETA-doped WS2 QDs with different DETA concentrations. The intensity of the N1s peak (∼399 eV) increases as the DETA concentration increases, indicating that N atoms are present in the DETA-doped WS2 QDs. The W4f region of XPS in the pristine WS2 QDs (Figure 5b) was analyzed and compared to that of the DETA-doped WS2 QDs (Figure 5c). For the pristine WS2 QDs, the peaks at about 32.5 and 34.7 eV were assigned to the W4f7/2 and W4f5/2 for 2H-WS2, respectively.21,27,38 After the DETA doping, two shoulders at ∼33.0 and ∼35.3 eV were observed in the W4f7/2 and W4f5/2 peaks, respectively (Figure 5c). The former and latter shoulders in the W4f regions were deconvoluted and assigned to the W−N bond for W4f7/2 and W4f5/2, respectively.38 The deconvoluted N1s spectrum of the DETA-doped WS2 QDs is shown in Figure 5d, displaying peaks at ∼399.5 and ∼401.5 eV, which corresponds roughly to the N−W and N−O bond in WS2 nanosheets, respectively.38 From these observations, we suggest that DETA was successfully grafted to WS2 in the DETA-doped WS2 QDs because of the presence of the W−N bond in the W4f region and the N−W bonds in the N1s region. The deconvoluted peaks for the S 2p scan are displayed in Figure 5e. The peaks at ∼164 and 162 eV are attributed to the S2p1/2 and S2p3/2, respectively.21 In Figure 5b,c, it was found that the density of the W−N (W−S) bonds was increased (decreased) after the DETA doping. We suggest that the intensity ratio of the W−N to W− S bond is related to the degree of DETA doping because the doping leads to a switch from the W−S bond to the W−N bond. Figure 5f plots the relative intensity ratio of W−N bonds to total bonds in the W4f region (i.e., W−S bonds + W−N bonds) as a function of DETA concentration. The intensity ratio of the W−N bond to total bonds increases as the DETA concentration increases. Accordingly, the degree of DETA doping on the WS2 QDs increases from 0 to 17% as the DETA concentration increases from 0 to 160 nM. The I−V characteristics of DETA-doped WS2 QDs with different DETA concentrations are displayed in Figure 6a. These curves exhibit obvious nonlinear I−V characteristics, which are associated with the Schottky barrier between the interface of WS2 QDs and ITO. According to the Schottky barrier model, the barrier height qϕS can be extracted from the following equation.39,40 ÉÑ Ä ÑÑ ij qϕS yzÅÅÅÅ ij qV yz Ñ 2 I = A*T expjjj zzzÅÅÅexpjjj zzz − 1ÑÑÑ j kBT zÅÅ j nkBT z ÑÑ (1) k {ÅÇ k { ÑÖ
2H-phase since the S sites are enhanced owing to an overlap of two S atoms in the 2H structure.25,26 We therefore conclude that both of the pristine and DETA-doped WS2 QDs belong to the 2H atomic structure. Figure 3a,b shows the AFM images of
Figure 3. AFM images of (a) pristine and (b) DETA-doped WS2 QDs. Height profiles corresponding to the lines in (a) and (b) are shown in (c) and (d), respectively.
the pristine and DETA-doped WS2 QDs dispersed on the Si substrate, respectively. The height profiles of some random dots selected from the pristine and DETA-doped WS2 QDs are shown in Figure 3c,d, respectively. The average heights of these QDs were between 0.6 and 1.3 nm, corresponding to 1− 2 monolayers of WS2.27 Figure 4a shows the Raman spectra of WS2 nanoflakes, the pristine WS2 QDs, and the DETA-doped WS2 QDs. In the
Figure 4. (a) Raman spectra of WS2 nanoflakes, pristine, and DETAdoped WS2 QDs. (b) FTIR spectra of DETA, pristine, and DETAdoped WS2 QDs.
WS2 nanoflakes, the Raman peak at ∼354 cm−1 was attributed to the in-plane phonon mode E12g, and the peak at ∼421 cm−1 to the out-of-plane A1g phonon mode, which agreed roughly with the reported results for WS2 nanoflakes.28−30 The E12g and A1g modes in WS2 QDs are found to be blue-shifted and redshifted, respectively, in comparison with those in WS2 nanoflakes. In addition, the intensity of Raman signals was greatly reduced in the QD samples. The Raman shift and the reduced Raman intensity in QDs has been attributed to the decrease of the layer−layer interaction force in QD structures.21,31 Figure 4b displays the FTIR spectra of pristine and DETA-doped WS2 QDs as well as the DETA dopant. The absorption bands around 616, 929, 1096, and 1170 cm−1 appeared in both of the QDs (marked by dashed lines) and were assigned to the W−S vibration and S−S vibration.32,33 The absorption band around 1640 cm−1 in both of the QDs is
where A* is the Richardson constant (134 A cm−2 K−2 for ITO), q is the electronic charge, T is the temperature, kB is the Boltzmann constant, V is the applied voltage, and n is the ideality factor. Good fits using eq 1 for the measured I−V curves were obtained and are shown in Figure 6a by the solid lines. The Schottky barrier height increases by ∼100 meV as C
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Figure 5. (a) XPS survey spectra of WS2 QDs doped with the DETA concentrations of 0, 16, 80, and 160 nM. XPS spectra of W4f region for (b) pristine and (c) DETA-doped WS2 QDs. XPS spectra of (d) S 2p and (e) N1s region for DETA-doped WS2 QDs. (f) Relative intensity ratio of W−N bonds to W−S and W−N bonds versus the DETA concentration.
Figure 6. (a) I−V curves of DETA-doped WS2 QDs with different DETA concentrations. The solid lines represent the result of fits using Schottky barrier model. (b) Schottky barrier height as a function of DETA concentration. The line is a guide for the eye. Schematic energy band diagrams of WS2 QDs with different degrees of doping: (c) low doping concentration and (d) high doping concentration.
the DETA concentration increases from 16 to 160 nM (Figure 6b). We hypothesize that the Fermi level of WS2 QDs is downshifted toward the valence band when the DETA concentration is increased. This p-type doping effect can be demonstrated by the later gate-dependent conductance measurements. In this case, a more pronounced band bending in the band diagrams of WS2-QD/ITO contacts will be produced by the WS2 QDs with higher DETA concentrations
than those with lower DETA concentrations, as shown in Figure 6c,d. Thus, an increase in the Schottky barrier height will appear when the DETA concentration is increased. Similar modulation of the Schottky barrier height by chemical doping has also recently been reported in WS2/Au contacts recently.41 It is worth noting that Figure 6a only shows the results for DETA concentrations up to 160 nM. Further increasing the DETA concentration in WS2 QDs will lead to a negative D
DOI: 10.1021/acsphotonics.8b00941 ACS Photonics XXXX, XXX, XXX−XXX
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Figure 7. (a) The ID−VG curves of DETA-doped WS2 QDs with different DETA concentrations. (b) Threshold voltage as a function of DETA concentration. (c) Hole density of the DETA-doped WS2 QDs as a function of DETA concentration. (d) Mobility of DETA-doped WS2 QDs as a function of DETA concentration. The lines in (b)−(d) are guides for the eye.
Figure 8. (a) PL spectra and (b) PL quantum yield of DETA-doped WS2 QDs with varying dopant concentration. (c) PL decay transients and (d) PL decay time of DETA-doped WS2 QDs versus the DETA concentration. The lines in (b) and (d) are guides for the eye.
increase in current modulation and the positive shift in Vth reveal successful p-type doping of WS2 QDs using PLA with DETA. This p-type phenomenon is in good agreement with the previous analysis of the Schottky barrier height. The hole density p due to DETA doping can be estimated from the shift of Vth according to the following equation:42
differential resistance behavior as the bias voltage is around 1.3 V (Supporting Figure S3), which interferes with the Schottkybarrier characteristics and is beyond the scope of this study. Figure 7a displays the drain current ID as a function of the applied gate voltage VG for the WS2 QDs with different DETA concentrations. All the gate-dependent conductance measurements were performed in air at room temperature. The ID for the highest DETA concentration (160 nM) improves about 3 orders of magnitude as compared with the pristine sample. The current modulation (i.e., Ion/Ioff, where Ion and Ioff are the drain currents when the device is in the “on” and “off” states, respectively) of WS2 QDs increases as the concentration of DETA doping increases. The highest current modulation reaches ∼104 for the WS2 QDs with a DETA concentration of 160 nM. The threshold voltage (Vth) shifts toward the positive gate voltage after DETA doping, as displayed in Figure 7b. The
p = Cg ΔVth /q
(2)
where Cg is the capacitance of the gate (SiO2) per unit area, ΔVth is the difference of the threshold voltage, and q is the elementary charge. Cg (= 3.45 × 10−8 cm−2) can be estimated from Cg = ε0εr/d, where ε0 is the dielectric constant in a vacuum, εr is the relative dielectric constant of 3.9 for SiO2, and d is the oxide thickness. With ΔVth obtained from Figure 7b, the hole density p estimated from eq 2 is displayed in E
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Figure 9. I−V curves under illumination of light with different intensities for DETA-doped WS2 QDs with the concentration of (a) 16, (b) 80, and (c) 160 nM (under 405 nm laser). (d) Photocurrent of WS2 QDs with varying DETA concentration. The line is a guide for the eye.
Figure 8b shows the quantum yield η with varying DETA concentrations. η increases as the DETA concentration is increased from 0 to 48 nM, but it decreases when the DETA concentration is more than 48 nM. The maximum η of the DETA-doped WS2 QDs reaches 23%, which shows an enhancement by ∼74 times when compared to that of pristine WS2 QDs. The effects of doping on the PL of a WS2 monolayer have recently been studied, and a PL enhancement ranging from 2 to 25 has been observed.9,10 The enhanced ratio of the PL intensity in our case is higher than that of previous reported WS2 monolayer. Again, this result indicates the DETA doping of WS2 QDs by using PLA technique is successful. Time-resolved PL of DETA-doped WS2 QDs was also measured to study their lifetimes. Figure 8c shows the PL transients of DETA-doped WS2 QDs with varying DETA concentrations. The PL lifetime can be fitted by the stretched
Figure 7c. The hole density increased from 6.3 × 1011 to 3.0 × 1012 cm−2 when the DETA concentration was increased from 16 to 160 nM. The doping level is comparable to that of other TMD monolayers with p-type molecular doping.43,44 The fieldeffect mobility μ can also be extracted from the gate-dependent conductance measurements using the equation:45 ij dI yz L μ = jjj D zzz j dVG z CgWVD k {
(3)
where W and L are the channel width and length, respectively, and VD is the drain-source voltage. According to the above equation, the mobility of WS2 QDs with different DETA concentrations was estimated and is displayed in Figure 7d. The mobility of the WS2 QDs improves monotonically from 1.51 × 10−3 to 0.14 cm2 V−1 s−1 as the DETA concentration increases from 16 to 160 nM. The improvement in mobility could be attributed to the increase in the available hopping sites introduced by the DETA doping. The increased hopping sites facilitate the hopping probability and enhance the mobility in the DETA-doped WS2 QDs accordingly. On the basis of above results, we demonstrate that not only is DETA an efficient p-type dopant for MoS2 QDs, but also the synthesized technique using PLA for doping in WS2 QDs is effective. The PL spectra of DETA-doped WS2 QDs with varying dopant concentrations are presented in Figure 8a. The peak position of PL in WS2 QDs was found to be blue-shifted after DETA doping, which can be interpreted by the effect of p-type doping. With p-type doping, the PL intensities from the charged exciton recombination were reduced, whereas the intensities from neutral exciton recombination were enhanced.3,8 Thus, the PL peak in DETA-doped WS2 QDs shifted toward the high-energy side because the peak energy from neutral exciton recombination is larger than that from the charged exciton recombination. Similar blue shift of the PL peak after p-type doping have also been observed in monolayer WS2 or few-layer MoS2.14,30,46 In addition to the peak shift in PL, the PL intensity in WS2 QDs was found to enhance significantly after the DETA doping. From Figure 8a, we can estimate the quantum yield η of DETA-doped WS2 QDs.
β
exponential function: I(t) = I(0)e−(kt) , where I(0), k, and β are the PL intensity at t = 0, the decay rate, and the dispersive exponent, respectively. The solid lines in Figure 8c display the fitted results using the above equation, which is in good agreement with experimental results. The PL average lifetime τ using the stretched exponential function can be estimated by τ = (kβ)−1Γ(β−1), where Γ is the Gamma function.47 The dependence of the PL lifetime in DETA-doped WS2 QDs on the DETA concentration is displayed in Figure 8d. The τ of DETA-doped WS2 QDs increases with the increase with the DETA concentration from 0 to 48 nM. It was found that the increased τ is in parallel with the increase of η (Figure 8b). We deduced that the enhanced PL in DETA-doped WS2 QDs originated from the increased carriers due to the DETA doping since the increased τ is associated with the increase in carrier densities. The WS2 QDs with higher DETA concentrations can thus produce more carriers to WS2 QDs (Figure 8c), which provides more transition probabilities in PL and leads to higher PL enhancement. However, a decrease in η and τ was observed after the doping concentration exceeded 48 nM (Figure 8b,d). The dependence of the PL intensity or PL lifetime on the carrier concentration has been studied previously in other TMDs and semiconductors.48,49 A similar decrease of the PL F
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of 160 nM in vacuum and in air (under the dark condition). The current of the DETA-doped WS2 QDs in air is much larger than that in the vacuum of 11 mTorr. This indicates that the conduction of WS2 QDs in air is not an intrinsic property, but rather, it depends on the absorbents surrounding the WS2 QDs. Figure 10b−d displays the I−V characteristics of DETAdoped WS2 QDs without/with light illumination in the vacuum, O2, and N2 conditions, respectively. The Iph of the DETA-doped WS2 QDs under vacuum, O2, and N2 conditions were extracted and found to be 0.2, 2.5, and 0.6 μA, respectively, at a bias voltage of 5 V. Figure 10e shows the effects of different atmospheres on the Iph. Compared with air, the O2 gas contributes to Iph to some extent, but the vacuum and the N2 gas indicate negligible influence. Figure 11a−c shows the I−V characteristics of DETA-doped WS2 QDs without/with light illumination under different values of RH. The values of the Iph increase from 0.8 to 18 μA as RH increases from 30% to 70%, indicating that the Iph of DETAdoped WS2 QDs is very sensitive to the humidity of the environment, and the N2 gas has a negligible influence. Figure 11d shows the dependence of RH on DETA-doped WS2 QDs with different DETA concentrations. The increase of Iph as a function of RH was clearly observed. In addition, Iph increases from 0.8 to 52 μA at RH = 90% as DETA concentration increases from 16 to 160 nM. This reveals that DETA doping can greatly enhance photoconductivity in WS2 QDs, which is favorable for the applications in humidity sensors or photoconductors. From the above result, we suggest that water molecules are the main contributor to Iph since Iph in O2 (2.5 μA) only contributed a small portion to that in water molecules (11 μA). As schematically illustrated in Figure 12, we propose a mechanism to explain the NPC in our WS2 QDs. Under dark and vacuum conditions (Figure 12a), conduction in WS2 QDs is governed by hopping and/or tunneling of carriers through the QDs. This results in a low current in the WS2 QDs, as shown in Figure 10a. Recently, the water and/or oxygen molecules have been reported to readily adsorb on the sulfur surface of MoS2 due to the vacancy/edge of MoS2 or the polarity of the molecules.54−58 In our case, the presence of the adsorbed water and/or oxygen molecules on WS2 QDs is supposed to induce a trap state at the surface/defect sites because the edge/surface of WS2 QDs may contain a large density of surface states or defects.30 The absorbed water molecules on the surface could be ionized by trapping electrons from the WS2 QDs because of their strong electronegativity and lead to the following process:58
intensity or PL lifetime has been observed for high carrier concentrations and is explained by the Auger recombination process.48,49 We therefore suggest that Auger recombination processes are responsible for the drop off in η and τ as the DETA concentration reaches more than 48 nM, as shown in Figure 8b,d. Nevertheless, we cannot exclude the possibility that the PL intensity could be reduced due to the blocking of the PL by the residue DETA in DETA-doped WS2 QDs. In this situation, the PL intensity would also be quenched with the increase in DETA doping. Figure 9a−c shows photo I−V characteristics under laser illumination (wavelength = 405 nm) with different excitation powers for the WS2 QDs with DETA concentration of 16, 80, and 160 nM, respectively. Interestingly, an obvious decrease in the current was observed under illumination for all photo I−V characteristics. This phenomenon is known as NPC and has been observed in other nanomaterials such as InAs nanowires,50,51 ZnO QDs,52 and reduced graphene oxide.38 However, to our knowledge, NPC has never been reported before in WS2 materials. Figure 9d shows the photocurrent Iph as a function of DETA concentration for the illuminated power of 70 mW, where Iph in our case is defined as the absolute value of the difference between the current under illumination (Ilight) and that in the dark (Idark). The Iph increases monotonically as the DETA concentration increases. The NPC mechanism has been reported to be complex in nature, which may depends on the interface defect states and/ or sensitivity to the environmental atmosphere.50,52,53 To investigate the effect of the surrounding gases on the conduction of WS2 QDs, the I−V characteristics in different atmospheric conditions were studied. Figure 10a shows the I− V characteristics of the WS2 QDs with a DETA concentration
H 2O(g) + 2e− → H 2 (g) + O2 − (ad)
(4)
Similarly, the absorbed oxygen molecules could be ionized as follows: O2 (g) + 4e− → 2O2 − (ad)
(5)
These processes cause a significant increase in hole densities due to electron neutrality, and an increase in conductance in WS2 QDs is observed accordingly (Figure 12b). Upon illumination, electron−hole pairs are generated from the laser having photon energy higher than the band gap of the WS2 QDs. The photogenerated hot carriers are able to desorb the adsorbed oxygen ions and detrap the trapped electrons in WS2 QDs. Attributable to the valence band bending caused by the surface potential, the photogenerated holes (h+) will have a
Figure 10. (a) I−V characteristics of DETA-doped WS2 QDs (160 nM) in atmospheric and vacuum conditions (under dark condition). I−V characteristics of DETA-doped WS2 QDs (160 nM) in dark and under illumination in (b) vacuum; (c) N2; and (d) O2. (e) Photocurrent of DETA-doped WS 2 QDs under various gas atmospheres. G
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Figure 11. I−V characteristics of the DETA-doped WS2 QDs (160 nM) in dark and under illumination with different relative humidity (RH): (a) 30%, (b) 50%, and (c) 70%. (d) Photocurrents of DETA-doped WS2 QDs versus RH.
This explains the enhancement of NPC in DETA-doped WS2 QDs as the DETA concentration is increased (Figure 9d).
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Figure 12. Schematic representation illustrating negative photoconductivity (NPC) of WS2 QDs (a) in vacuum, (b) in air, and (c) in air under illumination.
tendency to migrate to the WS2 QDs surface, discharging the adsorbed oxygen ions through the process:58,59 2O2 − (ad) + 4h+ → O2 (g)
CONCLUSIONS
DETA-doped 2H-WS2 QDs have been successfully synthesized using PLA. The enhancement of the current modulation and positive shift of the threshold voltage in gate-dependent conductance measurements confirms successful p-type doping on WS2 QDs. The hole density and the field-effect mobility increase greatly as the DETA concentration increases. The PL intensity of WS2 QDs is enhanced as high as 74 folds after the introduction of DETA. From the I−V characteristics in dark and illumination, it can be seen that the WS2 QDs possess NPC, which enhances after DETA doping. The light-induced desorbing of water/oxygen molecules is responsible for the mechanism of NPC in the WS2 QDs. Overall, DETA doping on WS2 QDs plays an important role in controlling the optical and electronic properties in WS2 QDs and could be essential in the development of WS2-QD-based optoelectronic devices.
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EXPERIMENTAL SECTION Synthesis. WS2 ultrafine powder used for the fabrication of DETA-doped WS2 QDs was purchased from the Graphene Supermarket (U.S.A.). An optical parametric oscillator (OPO) laser (EKSPLA NT242A) delivering pulses with a duration of 10 ns and a repetition rate of 10 Hz was used as the laser excitation source. The laser wavelength was tuned in the range of 415−490 nm to investigate the dependence of production yield on the laser wavelength. WS2 powder was suspended in ethanol with varying DETA concentrations and was irradiated to the laser pulses on a rotational stage (with angular velocity of 80 rpm). The laser irradiation was controlled under the fluence of 2.58 J/cm2 for 30 min. The suspension product after PLA treatment was centrifuged at 6000 rpm for 60 min and then filtered using the syringe filter (Millipore, 0.22 μm pore size). For characterization, the WS2 QDs were further diluted by dialysis using a 0.1−0.5 kDa membrane for 48 h. Characterization. For the size analysis of the WS2 QDs, TEM was performed in a JEOL JEM-2100F system with an
(6)
as shown in Figure 12c. Concurrently, the photogenerated electrons will recombine with the conduction holes in the WS2 QDs, which will lead to a decrease in photoconductivity. The decreased densities of the conduction holes results in a reduction in the current under illumination and, thus, produce NPC. When the doping concentration of DETA is increased, the density of the surface states or defect sites increases since the p-type doping is associated with point defects.60 The increase in surface states or defect sites induces more water and/or oxygen molecules within the atmosphere to adsorb on the surface, producing more oxygen ions (or electron trapping) in WS2 QDs. In addition, the amine groups in DETA are positively charged which can attract more electrons via Coulombic interactions and enhance the electron trapping effect. When the DETA-doped WS2 QDs are illuminated by light, more adsorbing/desorbing effects of the adsorbed molecules occur, and the NPC effect is enhanced accordingly. H
DOI: 10.1021/acsphotonics.8b00941 ACS Photonics XXXX, XXX, XXX−XXX
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(5) Xu, S.; Li, D.; Wu, P. One-Pot, Facile, and Versatile Synthesis of Monolayer MoS2/WS2Quantum Dots as Bioimaging Probes and Efficient Electrocatalysts for Hydrogen Evolution Reaction. Adv. Funct. Mater. 2015, 25, 1127. (6) Sik Hwang, W.; Remskar, M.; Yan, R.; Protasenko, V.; Tahy, K.; Doo Chae, S.; Zhao, P.; Konar, A.; Xing, H. G.; Seabaugh, A.; Jena, D. Transistors with Chemically Synthesized Layered Semiconductor WS2 Exhibiting 105 Room Temperature Modulation and Ambipolar Behavior. Appl. Phys. Lett. 2012, 101, 013107. (7) Kim, M.-J.; Jeon, S.-J.; Kang, T. W.; Ju, J.-M.; Yim, D.; Kim, H.I.; Park, J. H.; Kim, J.-H. 2H-WS2 Quantum Dots Produced by Modulating the Dimension and Phase of 1T-Nanosheets for Antibody-Free Optical Sensing of Neurotransmitters. ACS Appl. Mater. Interfaces 2017, 9, 12316. (8) Wang, X.; Sun, G.; Li, N.; Chen, P. Quantum Dots Derived from Two-Dimensional Materials and Their Applications for Catalysis and Energy. Chem. Soc. Rev. 2016, 45, 2239. (9) Rivera, A. M.; Gaur, A. P. S.; Sahoo, S.; Katiyar, R. S. Studies on Chemical Charge Doping Related Optical Properties in Monolayer WS2. J. Appl. Phys. 2016, 120, 105102. (10) Li, Z.; Ye, R.; Feng, R.; Kang, Y.; Zhu, X.; Tour, J. M.; Fang, Z. Graphene Quantum Dots Doping of MoS2 Monolayers. Adv. Mater. 2015, 27, 5235. (11) Zhao, W.; Ribeiro, R. M.; Eda, G. Electronic Structure and Optical Signatures of Semiconducting Transition Metal Dichalcogenide Nanosheets. Acc. Chem. Res. 2015, 48, 91. (12) Yang, X.; Yu, H.; Guo, X.; Ding, Q.; Pullerits, T.; Wang, R.; Zhang, G.; Liang, W.; Sun, M. Plasmon-Exciton Coupling of Monolayer MoS2-Ag Nanoparticles Hybrids for Surface Catalytic Reaction. Mater. Today Energy 2017, 5, 72. (13) Lin, W.; Shi, Y.; Yang, X.; Li, J.; Cao, E.; Xu, X.; Pullerits, T.; Liang, W.; Sun, M. Physical Mechanism on Exciton-Plasmon Coupling Revealed by Femtosecond Pump-Probe Transient Absorption Spectroscopy. Mater. Today Phys. 2017, 3, 33. (14) Peimyoo, N.; Yang, W.; Shang, J.; Shen, X.; Wang, Y.; Yu, T. Chemically Driven Tunable Light Emission of Charged and Neutral Excitons in Monolayer WS2. ACS Nano 2014, 8, 11320. (15) Feng, B.; Liu, X.; Zheng, Y.; Xiao, Q.; Wu, N.; Wang, S. A Label-Free Fluorescent Probe for Hg2+ Based on Boron- and Nitrogen-Doped Photoluminescent WS2. RSC Adv. 2016, 6, 49668. (16) Osada, K.; Tanaka, M.; Ohno, S.; Suzuki, T. Photoinduced Charge Transfer from Vacuum-Deposited Molecules to Single-Layer Transition Metal Dichalcogenides. Jpn. J. Appl. Phys. 2016, 55, 065201. (17) Hu, P.; Ye, J.; He, X.; Du, K.; Zhang, K. K.; Wang, X.; Xiong, Q.; Liu, Z.; Jiang, H.; Kloc, C. Control of Radiative Exciton Recombination by Charge Transfer Induced Surface Dipoles in MoS2 and WS2 Monolayers. Sci. Rep. 2016, 6, 24105. (18) Mouri, S.; Miyauchi, Y.; Matsuda, K. Tunable Photoluminescence of Monolayer MoS2 via Chemical Doping. Nano Lett. 2013, 13, 5944. (19) Yang, L.; Majumdar, K.; Liu, H.; Du, Y.; Wu, H.; Hatzistergos, M.; Hung, P. Y.; Tieckelmann, R.; Tsai, W.; Hobbs, C.; et al. Chloride Molecular Doping Technique on 2D Materials: WS2 and MoS2. Nano Lett. 2014, 14, 6275. (20) Mak, K. F.; Shan, J. Photonics and Optoelectronics of 2D Semiconductor Transition Metal Dichalcogenides. Nat. Photonics 2016, 10, 216. (21) Yan, Y.; Zhang, C.; Gu, W.; Ding, C.; Li, X.; Xian, Y. Facile Synthesis of Water-Soluble WS2 Quantum Dots for Turn-On Fluorescent Measurement of Lipoic Acid. J. Phys. Chem. C 2016, 120, 12170. (22) Yuwen, L.; Zhou, J.; Zhang, Y.; Zhang, Q.; Shan, J.; Luo, Z.; Weng, L.; Teng, Z.; Wang, L. Aqueous Phase Preparation of Ultrasmall MoSe2 Nanodots for Efficient Photothermal Therapy of Cancer Cells. Nanoscale 2016, 8, 2720. (23) Yang, G. Laser Ablation in Liquids: Applications in the Synthesis of Nanocrystals. Prog. Mater. Sci. 2007, 52, 648.
operating voltage of 200 kV. The height and the surface roughness of the WS2 QDs were obtained using an AFM system (psia XE-100) in a tapping mode. Raman spectra were recorded using a micro Raman spectroscopy system with a laser excitation wavelength of 532 nm and a charge-coupled device (CCD) detector. FTIR spectra were obtained through a Jasco FTIR-4200 with a transmittance mode. XPS spectra were employed using a Thermo Scientific K-Alpha ESCA, equipped with a monochromatic Al Kα X-ray source at 1486.6 eV. The PL properties of the WS2 QDs were studied with the aid of the Horiba Jobin Yvon FluoroMax-4PL spectrometer. The absolute quantum yield of WS2 QDs was determined from PL spectra covering both excitation and emission region for both experimental and reference samples by using spectrometer FluoTime 300 (PicoQuant). The dark and photo I−V characteristics of the WS2 QDs were measured between two ITO contacts using Keithley 2400. The I−V characteristics was performed by drop casting the WS2 QDs onto electrodes, followed by drying in an oven with a temperature of 60 °C. To measure the gate-dependent conductance, the source/drain electrodes on a 100 nm SiO2/p++ Si substrate were defined by optical lithography, followed by Ti/Al deposition using an electron beam evaporator.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.8b00941.
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Photographs of WS2 quantum dots after PLA treatment and collected supernatant, production yield of WS2 QDs, I−V curves of DETA-doped WS2 QDs (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Chi-Tsu Yuan: 0000-0003-3790-9376 Ji-Lin Shen: 0000-0003-2881-712X Funding
This project was supported by the Ministry of Science and Technology in Taiwan under grant number MOST 106-2112M-033-009-MY3. Notes
The authors declare no competing financial interest.
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