# Modulation of Optical and Electrical Characteristics by Laterally Stretching DNAs on CVD-Grown Monolayers of MoS2

Authors: Guru P. Neupane; Minh Dao Tran; Hyun Kim; Jeongyong Kim
Journal: Journal of Nanomaterials
 (2017)

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## Abstract

Monolayer MoS2 (1L-MoS2) is an ideal platform to examine and manipulate two dimensionally confined exciton complexes, which provides a large variety of modulating the optical and electrical properties of 1L-MoS2. Extensive studies of external doping and hybridization exhibit the possibilities of engineering the optical and electrical performance of 1L-MoS2. However, biomodifications of 1L-MoS2 and the characterization and applications of such hybrid structures are rarely reported. In this paper, we present a bio-MoS2 hybrid structure fabricated by laterally stretching strands of DNAs on CVD-grown 1L-MoS2. We observed a strong modification of photoluminescence and Raman spectra with reduced PL intensity and red-shift of PL peak and Raman peaks, which were attributed to electron doping by the DNAs and the presence of tensile strain in 1L-MoS2. Moreover, we observed a significant enhancement of electric mobility in the DNA/1L-MoS2 hybrid compared to that in the pristine 1L-MoS2, which may have been caused by the induced strain in 1L-MoS2.


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## Body

## 1. Introduction

Monolayers of transition metal dichalcogenides (1L-TMDs) are atomically thin direct band gap semiconductors [1–4], which host two dimensionally confined exciton complexes such as neutral excitons, trions, and biexcitons. These excitons system predominantly provoke the optoelectronic characteristics of 1L-TMDs, particularly 1L-MoS2 [1–4]. Therefore, the manipulation of such exciton systems has been a key to modulate the optoelectronic characteristics of 1L-MoS2 [5–11]. Electrostatic gating [5], surface plasmon [6, 7], chemical doping [8, 9], and strain engineering [10, 11] are promising ways to influence the population of the excitons complexes, where reversible electrostatic tunability of neutral excitons to trions has been demonstrated. When 1L-MoS2 was hybridized with metallic nanoparticles, the excitation of localized surface plasmons provided the enhancement of the photoluminescence (PL) from MoS2 [6, 7]. In the chemical doping process, the charge transfer between the dopant molecules and 1L-MoS2 modulated the intrinsic optoelectronic characteristics [8, 9]. The enhancement or reduction of the PL intensity of 1L-MoS2 depended upon the nature of doping (n-type or p-type chemicals) [8, 9, 12]. During the n-type doping process, the increase of electron density resulted in a dominance population of trions over the neutral excitons and the intrinsically faster nonradiative recombination of trions lowered the PL efficiency of n-type 1L-MoS2 [9, 12]. The higher chance of exciton-exciton annihilation at higher carrier density is also responsible for reducing the PL intensity [13]. On the contrary, the p-doping of 1L-MoS2 caused an increase in the spectral weight of neutral excitons and in the PL intensity [9].Furthermore, strain engineering has been a consistent route for the modulation of the optoelectronic characteristics of 1L-MoS2. Several theoretical and experimental results have demonstrated that the application of uniaxial or isotropic strains can modulate the band gap of monolayer 1L-MoS2 and can even induce the phase change from the direct gap to an indirect gap and thus have a significant effect on the PL intensity of 1L-MoS2 [14–17]. It has also been suggested that strain engineering of the band structure of MoS2 could be useful for increasing the carrier mobility of MoS2 [15, 18].While there have been many efforts to create hybrid structures of TMDs for enhanced physical and chemical characteristics [19–22], the hybridization of biomaterials and TMDs is relatively rare. Recently, ultra-high sensitivity field-effect transistor (FET) sensor using DNA-TMD system has been demonstrated [23], and DNA inducing doping effect on thick TMDs materials have been introduced [24, 25]. Biomolecules and TMD materials hold significant potential for the development of extremely small devices with increasingly complex functionality, which could open various applications in bioelectronics. Particularly, 1L-TMDs display the strong optical emission due to their direct band gap nature [1–4], and thus biohybridization of 1L-TMDs suggests promising optoelectronic applications. However, DNA-TMD hybrid structure using 1L-TMDs and the consequent modulation of optical properties of 1L-TMDs was not studied so far.In this study, we present a hybrid structure of laterally stretched DNA strands on 1L-MoS2. Significant modifications in PL intensity and Raman spectra of 1L-MoS2 were observed, which indicated the n-doping effect and tensile strain caused by hybridization with stretched DNA strands. Furthermore, we observed an enhancement in the carrier mobility of DNA/1L-MoS2, of almost one order of magnitude, attributed to the tensile strain. Our results demonstrate a diverse engineering possibility of bio-TMD hybrid structures for promising optoelectronic applications.


## 2. Experiment

First, the glass substrates (22 × 40 × 1 mm3) were cleaned with 1 M-KOH. Next, in order to obtain a charged and hydrophilic surface, the acetone and distilled water in ultrasonic bath were treated with piranha and RCA solution [26, 27]. The CVD-grown [28] 1L-MoS2 was transferred onto these treated substrates using the wet transfer technique [29]. We eliminated the polymethyl methacrylate (PMMA) residues by continuously cleaning with acetone, IPA, and ethanol. This 1L-MoS2 was subjected to oxygen plasma treatment using radio frequency (RF) plasma cell to obtain a hydrophilic 1L-MoS2 surface. The oxygen plasma treatment was performed under an operating pressure of 1.3×10-3 Pa, RF power of 50 W, and 5 sccm flow rate of oxygen gas for 5 s.We purchasedλ-DNA (0.3 μg/μL, Thermo Scientific Corp.) and YOYO-1 (1 mM solution in dimethyl sulfoxide, Invitrogen Corp.). For the fluorescence visualization of DNA stretching, we formed a YOYO-1 dye stained λ-DNA solution by mixing the exact amount of dye in the λ-DNA solution [30], and we centrifuged the mixture for 15 min under 3000 rpm at room temperature for effective intercalation. Finally, 50 μL of the DNA-dye solution was spin coated over the surface of the modified glass substrate containing plasma treated 1L-MoS2 with a rotating speed of 4550 rpm for the first 20 s and 5750 rpm for an additional 10 s for optimal dispersion of stretched DNA [30].For electrical measurements, 1L-MoS2 was transferred onto the modified SiO2/Si substrates using wet transfer techniques [29]. Using the normal photolithography technique, the source and drain electrodes of the devices were fabricated [31, 32]. For these electrodes, a 50 nm thick Au layer with Cr/5 nm was deposited by applying the electron beam evaporation technique [31, 32]. Moreover, the device was treated with oxygen plasma under the same conditions. DNA-YOYO-1 solution was spin coated over the device with similar experimental parameters, as described above. The electrical transport characteristics of the fabricated 1L-MoS2 device with pristine state and after DNA stretching were tested in ambient conditions.We used a lab-made inverted laser confocal microscope (LCM) system combined with a spectrometer for the measurements of confocal PL, Raman, and low temperature PL [12, 33]. Our samples were excited with a 514 nm wavelength line of an Ar-ion laser and a 633 nm wavelength line of a He-Ne laser for the Raman and PL measurements. We used an oil-immersion objective lens with a 1.4 numerical aperture for focusing the laser light on the samples and the same objective was used to collect the PL and Raman signals from the samples. The lateral resolution of the PL imaging and spectroscopy was estimated to be ~500 nm [34, 35]. The typical laser power values for the PL and Raman measurements were 70 μW and 300 μW, respectively. For exact comparison between pristine and DNA-stretched 1L-MoS2, we used the same experimental parameters for each set of the PL and Raman measurements. The fluorescence images of DNA-stretched 1L-MoS2 were taken with 436 nm line of Hg lamp. The FET measurements were performed using a four-probe station equipped with a current detector (Source Meter 2400, Keithley) at ambient condition.


## 3. Results and Discussion

While the deposition of DNA solution was previously used for DNA/TMD hybridization [23, 24], here we laterally stretched strands of DNA molecules on 1L-TMDs. Figure 1 shows the schematic for stretching the DNA molecules on the glass substrate and MoS2 surface. Glass substrates treated with piranha and RCA solution, as described in the experimental section, developed negative charges on the surface [26, 27]. Subsequently, 1L-MoS2 sample was transferred onto the modified substrate. Formation of hydrogen bonds between the alkyl functional group of YOYO-1 dye molecule and the amine functional group contained in the DNA molecule induced the intercalation of YOYO-1 molecules in the DNA [30]. To obtain a hydrophobic MoS2 surface, we performed oxygen plasma treatment. We ensured that the oxygen plasma treatment did not affect the optical characteristics of 1L-MoS2. (See Figure S1 in Supplementary Material available online at https://doi.org/10.1155/2017/2565703 for PL mapping images, PL spectra, and Raman spectra before and after oxygen plasma treatment.) Finally, YOYO-1 stained DNA molecules were spin coated. The two competitive forces of centrifugal force and the electrostatic force resulted in lateral stretching of the DNAs on the substrate and on the 1L-MoS2 surface as well. In more detail, because YOYO-1 stained DNAs consist of positively charged amine groups and the substrate is negatively charged by piranha and RCA treatment, adhesive electrostatic force between the substrate and DNA molecules is developed. On the other hand, the spin coating process provides the centrifugal force applied to DNA strands and resulted in stretching of DNA molecules in specific orientation. The proper selection of spin coating parameters, such as rotating speed and time supports the good stretching of the DNA strands. Excessive spin speed can cause the shearing of DNA strands while low speed can cause the dispersion of coiled and aggregated DNA strand [30].Figure 1
Schematic illustration of DNA stretching on 1L-MoS2 surface. The surface of the glass substrate was negatively charged by piranha and RCA treatment (see the text) and YOYO-1 stained DNA strands were spin coated on the sample, which resulted in lateral stretching of DNA strands on 1L-MoS2.Figure2 displays a bright-field microscope image and the fluorescence image of laterally stretched DNAs on CVD-grown 1L-MoS2. The DNA strands were not visible in bright-field microscope image, owing to a significantly small diameter (~2 nm) of single DNA strands. However, the presence of strands of DNAs on the sample was clearly revealed in the fluorescence image by the fluorescence emitted from YOYO molecules incorporated in DNA chains, particularly between the triangular grains of CVD-grown 1L-MoS2. Interestingly YOYO-1 fluorescence of DNAs is completely quenched in the region of MoS2 grains. The absence of YOYO-1 fluorescence is likely to be caused by the charge transfer or the energy transfer from YOYO-1 to MoS2.Figure 2
(a) Bright-field optical image of the sample. (b) Epifluorescence image of YOYO-1-stained DNA strands stretched on 1L-MoS2. 436 nm line of Hg lamp was used for fluorescence excitation. Scale bars indicate 10 μm.
(a)
(b)We present the results of the PL spectra mappings of pristine 1L-MoS2 and 1L-MoS2 with laterally stretched DNAs using a 633 nm wavelength laser in Figures 3(a)–3(c). The representative PL intensity maps, PL spectra, and PL peak position maps are shown in Figures 3(a), 3(b), and 3(c), respectively. The laser light with a 633 nm wavelength is likely to be absorbed by 1L-MoS2 and not by YOYO-1 dye, and therefore, the PL of 1L-MoS2 can be studied without being interfered by the fluorescence of YOYO-1. The PL intensity of 1L-MoS2 was found to be drastically reduced after the stretching of DNAs. The averaged PL spectra in Figure 3(b) display ~50% of reduction in the PL intensity of 1L-MoS2 owing to the stretching of DNA molecules. The peak position maps of A excitons in Figure 3(c) show the significant red-shift of A exciton peak by ~16 nm in the DNA-stretched 1L-MoS2. The red-shift of the PL peak has been interpreted as a result of an increase in the trion spectral weight originating from the increase of electron density [9, 12]. The reduction of PL intensity and red-shift of PL peak suggests the n-doping effect in 1L-MoS2 after the lateral stretching of DNAs on 1L-MoS2. We did not observe any noticeable defect passivation or increase in 1L-MoS2 caused by DNA stretching in low temperate PL spectra as shown in Figure 3(d), which suggests that the likely change of defect states of 1L-MoS2 after the DNA stretching is not responsible for the observed modification of PL spectra.Figure 3
(a) PL intensity maps of pristine and DNA-stretched 1L-MoS2 with the laser excitation of 633 nm line. (b) Averaged PL spectra, (c) peak position maps of A exciton peaks, (d) temperature-dependent PL spectra, and (e) averaged Raman spectra obtained from pristine 1L-MoS2 and DNA-stretched 1L-MoS2. Scale bars indicate 5 μm.The averaged Raman spectra in Figure3(e) show the red-shift on both the E12g and A1g Raman modes in 1L-MoS2 by 5 and 3 cm−1, respectively, after DNA stretching. The red-shift of the E12g mode for DNA/MoS2 systems was previously observed [23, 24] and the observed red-shift in the peak position of the E12g Raman mode indicates the presence of tensile strain in 1L-MoS2 with lateral stretching of DNAs, which amount to ~1% according to previous empirical results [10, 36]. The red-shift of A exciton PL peak and A1g Raman mode in DNA-stretched 1L-MoS2 can be caused by electron doping in 1L-MoS2 [9, 12]. The A1g Raman mode shift of 3 cm−1 corresponds to an increase of ~1.4 × 1013/cm2 in the electron density [37]. The presence of a negatively charged phosphate group in the backbone of the DNA may be the origin of such electron doping effect [24, 25]. In addition, the tensile strain may be responsible for the observed red-shift of PL peak energy in our DNA/1L-MoS2, since tensile strain is known to reduce the bandgap of 1L-MoS2[10, 36] and thus we expect stretched DNA molecules could cause the band gap modulation of 1L-MoS2. Unfortunately, we cannot precisely separate the effects of doping and strain because observed PL peak shift is the result of both effects. However, as the A1g mode and E12g mode are known to represent the strain and the electron density of 1L-MoS2 independently, our results suggest the coexistence of electron doping effect and tensile strain in 1L-MoS2 owing to the stretching of DNA strands.We found that modulation of PL and Raman spectra was dependent upon the density of DNAs stretched on 1L-MoS2, being less if a less number of DNAs were stretched. As displayed in Figure S2, we observed ~11 nm of red-shift and ~40% reduction in PL intensity of A exciton PL peak and 2.8 and 4.0 cm−1 of A1g and E12g Raman peaks from the region with seemingly less density of stretched DNAs than in the region shown in Figure 2. This result suggests some degree of tunability of optical properties of DNA/1L-MoS2 hybrid by controlling the density of stretched DNA molecules.We studied the effect of DNA stretching on the electrical transport of 1L-MoS2 using a back-gated FET configuration. Figure 4 shows the plot of source-drain current versus gate bias (Ids–Vg) with a source-drain voltage of 1 V for pristine 1L-MoS2, after the stretching of DNA strands. First, we note that the threshold voltage shifted by −68 V as a result of the DNA stretching. Moreover, we extracted the field-effect electron mobility (μFE) values from the transport curves according to the equation μFE=dIds/dVds(L/WCiVds), where Ci is the capacitance between the MoS2 channel and the silicon layer per unit area and L and W are the length and width of the channel, respectively [32]. μFE values of 2.5 and 22.4 cm2 V−1 s−1 were estimated for the pristine state of 1L-MoS2 and for our DNA/1L-MoS2 device, respectively, indicating a significant enhancement in the electron mobility on 1L-MoS2 as a result of laterally stretched DNA strands. The electron mobility can be enhanced by reducing the structural defects or enhancing the carrier density of 1L-MoS2 [38, 39]. Furthermore, theoretical studies have predicted that introducing tensile strain to 1L-MoS2 can increase the field-effect mobility owing to a decrease in the effective mass of the electrons [15, 18]. Considering that a low temperature PL shows limited difference in terms of defect-oriented emission (Figure 3(e)), we believe that there exists no significant reduction in defect density with DNA stretching. We noticed the slight decrease of on-off current ratio in the DNA/1L-MoS2 FET, which was in the order of 104 in the hybrid structure while it was in the order of 105 in the pristine state. The observed decrease of on-off ration could originate from the reduction of bandgap [40] that may have occurred by the introduction of tensile strain by DNA stretching.Figure 4
FET transport characteristics of pristine 1L-MoS2 and DNA-stretched 1L-MoS2. Inset shows the optical image of the FET device. Scale bar indicates 20 μm.


## 4. Conclusion

We prepared a bio-TMD nanostructure by laterally stretching the DNAs on 1L-MoS2. We observed a strong modification of photoluminescence and Raman spectra with reduced PL intensity and red-shift of the PL peak and Raman peaks in 1L-MoS2, which were attributed to electron doping by the DNAs and the presence of tensile strain in 1L-MoS2. We also observed a significant enhancement of electric mobility in DNA/1L-MoS2 heterostructure compared to that in pristine 1L-MoS2, which is likely to be caused by the induced strain in 1L-MoS2. The optical characteristics of the stretched DNAs on 1L-MoS2 were also strongly modified, originating from the effects of charge transfer and tensile strain. The carrier mobility of 1L-MoS2 has significantly increased with stretched DNAs on 1L-MoS2, suggesting promising optoelectronic application of bio-TMD hybrid nanostructures.



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