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Direct Patterning of Zinc Sulfide on a Sub-10 Nanometer Scale via Electron Beam Lithography Mohammad S. M. Saifullah,*,† Mohamed Asbahi,*,† Maryam Binti-Kamran Kiyani,† Sudhiranjan Tripathy,† Esther A. H. Ong,‡ Asadullah Ibn Saifullah,‡,§ Hui Ru Tan,† Tanmay Dutta,†,∥ Ramakrishnan Ganesan,⊥ Suresh Valiyaveettil,*,‡ and Karen S. L. Chong† †

A*STAR (Agency for Science, Technology, and Research), Institute of Materials Research and Engineering, 2 Fusionopolis Way, #08-03 Innovis, Singapore 138634 ‡ Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 § Hills Road Sixth Form College, Hills Road, Cambridge CB2 8PE, United Kingdom ∥ Department of Electrical and Computer Engineering, National University of Singapore, 21 Lower Kent Ridge Road, Singapore 117576 ⊥ Department of Chemistry, Birla Institute of Technology & Science, Pilani−Hyderabad Campus, Jawahar Nagar, Shameerpet Mandal, Hyderabad 500 078, Telangana, India S Supporting Information *

ABSTRACT: Nanostructures of metal sulfides are conventionally prepared via chemical techniques and patterned using self-assembly. This poses a considerable amount of challenge when arbitrary shapes and sizes of nanostructures are desired to be placed at precise locations. Here, we describe an alternative approach of nanoscale patterning of zinc sulfide (ZnS) directly using a spin-coatable and electron beam sensitive zinc butylxanthate resist without the lift-off or etching step. Time-resolved electron beam damage studies using micro-Raman and micro-FTIR spectroscopies suggest that exposure to a beam of electrons leads to quick disappearance of xanthate moieties most likely via the Chugaev elimination, and further increase of electron dose results in the appearance of ZnS, thereby making the exposed resist insoluble in organic solvents. Formation of ZnS nanocrystals was confirmed by high-resolution transmission electron microscopy and selected area electron diffraction. This property was exploited for the fabrication of ZnS lines as small as 6 nm and also enabled patterning of 10 nm dots with pitches as close as 22 nm. The ZnS patterns fabricated by this technique showed defect-induced photoluminescence related to sub-band-gap optical transitions. This method offers an easy way to generate an ensemble of functional ZnS nanostructures that can be arbitrarily patterned and placed in a precise way. Such an approach may enable programmable design of functional chalcogenide nanostructures. KEYWORDS: zinc sulfide, nanofabrication, electron beam lithography, resists, xanthates, single source precursors, photoluminescence

M

radius. Zinc sulfide (ZnS) is one of the most important II−VI wide-band-gap semiconductors,4 which exists in two commonly available allotropeszinc blende (cubic form) and wurtzite (hexagonal form). The cubic form is the stable low-temperature polymorph, whereas the latter is the high-temperature phase. In both of these forms, the coordination geometry at zinc is

etal chalcogenides, mainly consisting of sulfides, selenides, and tellurides, are an interesting class of materials that find applications in various domains including optoelectronics, solar cells, fuel cells, sensors, memory devices, bioimaging, thermoelectrics, photodetectors, and photocatalysis.1−3 One of the fascinating characteristics exhibited by the semiconducting metal chalcogenides is the size-dependent band gap due to the quantum confinement effect, which is predominant when the size of the chalcogenide nanocrystal is comparable to or smaller than the exciton Bohr © 2017 American Chemical Society

Received: June 6, 2017 Accepted: September 22, 2017 Published: September 22, 2017 9920

DOI: 10.1021/acsnano.7b03951 ACS Nano 2017, 11, 9920−9929

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Cite This: ACS Nano 2017, 11, 9920-9929

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images from the ZnS further confirmed these findings, where sub-band-gap emissions from defect levels are detected.

tetrahedral. The band gap is in the range of 3.5 and 3.9 eV for zinc blende and wurtzite, respectively. ZnS has a high refractive index, a high transmittance in the visible range, and its nanoparticles show excellent size-dependent electrical and optical properties owing to the quantum confinement effect.5−8 Although individual nanoparticles show interesting properties, they find little application due to the lack of functionality and difficulty in handling. The transition from synthesis of individual nanoparticles to the preparation of their ensemble is driven by the enhanced functionality observed due to coupling of electronic, optical, or magnetic properties of individual nanoparticles. Traditionally, this is achieved by self-assembly that harnesses the nanoscale forces and provides a simple, elegant, and, more importantly, inexpensive method to generate the ensemble of nanoparticles.9−12 While bottom-up process such as self-assembly has its own advantages, it also suffers from the problem of size of area over which such an ensemble can be realized. This issue has been somewhat alleviated using a combination of top-down approaches to design a template over which self-assembly can be carried out over fairly large areas.13 Top-down approaches such as optical, electron beam, and nanoimprint lithographies, or a combination of these, offer a better method to pattern metal sulfides on a nanoscale as they have the potential to provide flexibility in design and area required for patterning. Although attractive, the work on direct lithographic patterning of sulfides is still in its infancy due to difficulties in finding suitable precursors to pattern them. Typical precursors used to lithographically pattern metal sulfides are thiolate,14 xanthate,15 and long-chain metal anion−alkyl ammonium complexes.16 Thiolates and xanthates are usually mixed with a polymer to improve the film-forming ability during spin-coating and, in some cases, to suppress the crystallization of these salts.14,17 When this mixture is lithographically patterned14 and/or heated at temperatures around 200 °C, it leads to breakdown of the precursors to their respective nanoscale metal sulfides that are embedded inside the polymer matrix.17 The problem with adding polymers to precursors and dealing with large molecular complexes is that the net ceramic yield becomes very low. In this work, we describe a combination of a top-down approach using electron beam nanolithography with a single source molecular resist in order to pattern arbitrary size and shapes of functional ZnS nanostructures. The technique allows us to position these structures accurately over large areas on a sub-10 nm scale without the need of lift-off or etching steps. In order to minimize the problem of low ceramic yield, a rational approach to resist design was undertaken by studying the effect of length of organic chain on resist properties. It was concluded that zinc butylxanthate, a single source precursor, provides the best balance between resist solubility, electron beam exposure, and development characteristics. Furthermore, when this resist was exposed to an electron beam, it decomposed in situ to produce ZnS. A development step removes the unpatterned resist, leaving behind nanopatterned ZnS. Thermal and optical characterization of zinc butylxanthate was carried out to understand the evolution of ZnS phase during heat-treatment and electron beam exposure. Due to lower scattering efficiency of visible Raman excitations, ultraviolet near-resonant Raman excitation (325 nm) was preferred in the analysis of such patterned ZnS with a sub-micrometer film thickness. The optical phonons in Raman spectra under thermal and electron beam treatments show the presence of zinc blende phase in these ZnS structures. The microscopic photoluminescence

RESULTS AND DISCUSSION Metal xanthates have the general chemical formula M(S2COR)n, where M is a metal with an oxidation state of n and R is the alkyl group. These compounds have proven to be versatile single source precursors for preparing metal sulfide films and nanoparticles. Their principal advantage comes from the fact that the metal atom is directly bonded to sulfur and that they readily undergo thermal decomposition at fairly low temperatures to give metal sulfides. The thermal behavior of metal xanthates is closely related to that of metal dithiocarbamates [M(S2CNRR′)n]. However, the former has a better film-forming ability. A preliminary study undertaken to compare the film-forming ability of spin-coated zinc ethylxanthate and zinc diethyldithiocarbamate suggested that the former gives smooth films, whereas the latter forms highly crystalline and nonuniform films. Thus, xanthates became the material of choice for this work. Zinc acetate was reacted with potassium alkylxanthate (R = ethyl, propyl, isopropyl, butyl, and pentyl) in the molar ratio of 1:2 to obtain zinc alkylxanthate as shown in the reaction below.

A white precipitate of zinc alkylxanthate was formed immediately. It was washed with deionized water and dried overnight in a vacuum oven. Resists of zinc alkylxanthates were prepared by dissolving them in chloroform and adding anisole solvent. Anisole prevents quick evaporation of chloroform and thereby provides smooth films after spin-coating. Electron beam exposure response behavior of various zinc alkylxanthate resists was studied by exposing predefined lines at various doses. The heights of these lines were measured using a scanning probe microscope (SPM), and their normalized values were plotted (Figure 1) from which sensitivity and contrast of respective resists were calculated (Table 1). Sensitivity of a resist is defined as an exposure that provides thickness of the remaining film equal to 50% of its original value. For negative tone resists, contrast is defined as γ = |log(D1/D0)|−1, where D0 and D1 correspond to electron doses at 0 and 100% of remaining film thickness, respectively. It is observed that sensitivity and contrast of the zinc alkylxanthate resists show no particular trend with the increasing length of alkyl chain of xanthate. However, notable increase in the values of sensitivity and contrast was observed for zinc pentylxanthate (sensitivity = 26 mC/cm 2 ) and zinc isopropylxanthate (γ = 11.2), respectively. The value of these results must be examined against the backdrop of solubility of these materials. Zinc alkylxanthates with longer alkyl chain length show much better solubility in organic solvents, especially chloroform. It was observed that lower solubility of compounds also leads to poorer postexposure development characteristics such as the presence of residue on the surface. The solubility of alkylxanthates is more pronounced from zinc butylxanthate 9921


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Figure 1. Electron beam sensitivity curves for various zinc alkylxanthate resists.

Table 1. Electron Beam Sensitivity and Contrast Values for Various Zinc Alkylxanthate Resists resist zinc zinc zinc zinc zinc

ethylxanthate propylxanthate isopropylxanthate butylxanthate pentylxanthate

sensitivity (mC/cm−2)

contrast (γ)

35 37 32 46 26

3.1 2.4 11.2 1.8 3.8

onward.18 This observation must be balanced with the fact that a longer alkyl chain results in greater loss of material during the thermal step and thus leads to smaller ceramic yield. In order to balance these competing factors, zinc butylxanthate (sensitivity = 46 mC/cm2, γ = 1.8) was chosen as the resist material for further study. For the sake of comparison, hydrogen silsesquioxane (HSQ), a commonly used high-resolution electron beam resist, shows sensitivity and contrast values of ∼2.5 mC/cm2 and ∼13, respectively, when exposed at 100 kV and developed using a salty developer.19 The thermal characteristics of the zinc butylxanthate resist were determined using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) studies (Figure 2). The TGA curve depicts a two-step mass loss beginning at approximately 110 °C. The first step leads to a rapid decomposition with a material loss of 65.1%. Decomposition begins at a temperature of 110 °C and is virtually complete by 180 °C. The second step is a minor mass loss of 7.5% that happens between 250 and 350 °C (Figure 2a,b). Therefore, the net mass loss is 72.6%. Thus, the obtained ceramic yield is 27.4%, which corresponds very well with the theoretical ceramic yield of 26.8% for ZnS obtained from decomposition of zinc butylxanthate. On the other hand, the DSC result suggests that there is a melting (endotherm) of zinc butylxanthate followed by its decomposition (exotherm) during the first step of mass loss (Figure 2c). This result is significant as it shows that, although zinc butylxanthate is stable at room temperature, it becomes highly unstable when exposed to temperatures above 110 °C. In order to understand the phase changes that are happening in zinc butylxanthate during heating, thick films on silicon

Figure 2. (a) TGA, (b) first derivative of TGA trace, and (c) DSC plots of zinc butylxanthate.

substrate were heat-treated at various temperatures for 1 h inside a sealed quartz tube furnace with continuous nitrogen gas flow (flow rate = 10 L/min). X-ray diffraction (XRD) study (Figure 3) shows that the as-coated zinc butylxanthate film is amorphous at room temperature. However, at temperatures 250 °C and above, cubic ZnS (i.e., zinc blende) phase appears. Although the minimum heat-treatment temperature used was 250 °C due to the technical limitations of the tube furnace, it is likely that heat-treatment below this temperature may also yield cubic ZnS phase, albeit impure. The XRD studies were complemented with Raman studies in order to have better understanding of crystalline properties of thermally obtained cubic ZnS. Due to wider band gap (∼3.7 eV), Raman measurements using 488 and 514 nm excitation wavelengths showed lower scattering efficiency on silicon substrates, as optical phonons from underlying bulk silicon dominated the Raman spectra. In this context, we have used 325 nm Raman excitations to record the near-resonant Raman spectra from our samples. Due to a higher scattering efficiency and shallow probing depth, UV Raman measurements show distinguished ZnS optical phonons addressing the nature of ZnS formed on the silicon substrates. Based on our investigations of the zinc blende phase detected from the XRD, we have shown Raman 9922


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Brillouin zone (Γ point). The optical phonon peaks located at 348 cm−1 in Figure 4 are consistent with previous Raman data for zinc blende ZnS phase and corroborates very well with the XRD observations. In addition, a much broader mode around 277 cm−1 related to the TO phonon mode is observed in some samples. These modes are commonly referred as first-order TO and LO modes of cubic ZnS. Other weaker features detected in the Raman spectra represent a combination of optical and acoustic modes and their overtones. Due to near-resonant Raman conditions, the broader peak around 693 cm−1 is associated with the second-order LO phonon of ZnS (LO2). A small peak shift of the LO mode toward higher energy side at higher processing temperatures might result from an increase in grain size of ZnS. Electron dose-dependent chemical changes taking place in a thick zinc butylxanthate resist were studied using 50 μm wide patterned disks. As we reduce the physical dimensions of structures by electron beam patterning, the XRD technique is less sensitive for the investigation of crystalline properties. Therefore, micro-Raman technique was used as an alternative to investigate patterned features, and it was supplemented with micro-FTIR (Fourier transform infrared spectroscopy) to gain a better understanding of the ongoing chemical changes (Figure 5). The micro-Raman studies were conducted using a 325 nm excitation on the patterns subjected to different electron doses. It was observed that with an increasing electron dose, zinc butylxanthate was converted to ZnS. Figure 5a shows the evolution of first- and second-order optical phonons of ZnS with an increase in electron beam dose. The spectral features within the patterns are down-shifted compared to the samples subjected to thermal treatments, where first-order LO and second-order LO2 modes appear at 344 and 691 cm−1, respectively. The intensity of the LO2 mode with respect to LO increases in samples subjected to higher beam doses. Furthermore, due to such a resonant process, a much weaker combination mode appeared around 420 cm−1 comprising an optical mode plus an acoustic mode. The change of phonon dispersion related to crystallite size confinement effect led to significant phonon softening and broadening in this case when compared to phonon peaks of bulk ZnS. Spectral broadening is also seen at the lower energy side around 250−300 cm−1 due to presence of TO phonon and surface vibrations. However, with the gradual increase in electron beam dose from 29.2 to 39.2 mC/cm2, a higher energy LO phonon shift was observed. This behavior could be related to an increase in grain size of ZnS at higher electron beam doses. Due to the nature of scattering process, the intensity ratio of the first- and second-order LO modes can be correlated to the structural modifications of ZnS in thermal or electron beam treated samples. The integrated peak intensity ratios of the LO/LO2 modes seen in these samples are different due to the variation of ZnS grain size under variable processing conditions. Due to quantum confinement effect in nanosize ZnS, a smaller crystallite size leads to a change in the band gap and optical phonon dispersion. Both phonon confinement effect and latticemismatch-induced strain controls the amount of LO phonon peak shifts in these samples. An increase in out-going resonance due to Fröhlich interaction in such electron beam treated samples led to a relative enhancement of LO2 intensity. The corresponding micro-FTIR data of electron beam exposed discs is plotted in Figure 5b. Table 2 lists the characteristic infrared absorption peaks of zinc butylxanthate resist.23,24 It is interesting to note that although minimal

Figure 3. XRD data of zinc butylxanthate resist spin-coated on a silicon substrate and heat-treated at various temperatures for 1 h. The phase obtained matches well with cubic ZnS (JCPDS data file 05-0566). The relative intensity of peaks for this phase is plotted as vertical lines at the bottom.

spectra from thermally treated samples in Figure 4. The spectra under 325 nm wavelength excitations clearly show strong

Figure 4. UV Raman spectra of the samples under 325 nm excitations. The spectra recorded from zinc butylxanthate resist spin-coated on a silicon substrate and heat-treated at various temperatures for 1 h show the presence of strong first-order LO phonon peaks at 348 cm−1 and weaker second-order LO2 modes at 693 cm−1.

Raman peaks around 349 cm−1, with much broader and weaker peaks at 440 and 693 cm−1 resolved with an increase in thermal treatment temperatures. To avoid UV laser-induced degradation and sample heating, the spectra were acquired with shorter acquisition times in the order of seconds. All spectra show background silicon substrate peaks at 520 cm−1 due to a lower thickness of ZnS. Based on the overview of zinc blende ZnS phase with the point group Td (43m), three optical phonons are dominant with T2 = Γ15 symmetry.20−22 The optical modes are doubly degenerate TO, and single LO phonons are usually referred as T2(TO) and T2(LO) modes near the center of the 9923


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Figure 5. (a) Raman spectra evolution with electron beam exposure of zinc butylxanthate resist. An increase in intensity ratio of second-order LO2 mode with respect to first-order LO mode of ZnS is seen with an increase in electron dose. (b) Time-resolved FTIR spectroscopy of electron beam damage of zinc butylxanthate resist. The weak band at 960 cm−1 is shown by an arrow.

their small size, low crystallinity, and structural defects. Furthermore, presence of defects such as dangling bonds at the surface or interface of nanocrystals may also lead to additional energy levels in the band gap. Nanocrystalline ZnS is known to show photoemission when excited with short wavelength radiation. This has been attributed to various factors such as recombination of electrons from the energy level of sulfur vacancies with the holes from the valence band, surface states, and point defects.26−30 The photoemission from the electron beam patterned 50 μm wide patterned disks of ZnS was studied using fluorescent microscopy. A 405 nm diode laser in a confocal microscope was used to excite the patterned discs, and the emission from them was collected between 450 and 500 nm and 500−600 nm wavelength ranges (Figure 6b). It was observed that at a low electron dose of 25.2 mC/cm2, weak photoluminescence was seen in both ranges, suggesting the nucleation of a small amount of ZnS nanocrystals with structural defects. However, with an increasing electron dose, the intensity of luminescent transitions increases, implying that the proliferation of ZnS nanocrystals, particle size, surface states, and surface point defects were probably the driving forces behind photoluminescence. At doses close to 120 mC/ cm2, the photoluminescence falls in both 450−500 and 500− 600 nm wavelength ranges. This may be attributed to the electron beam aided reduction of point defects in ZnS nanocrystals; that is, the energy from the electron beam resulted in the improved perfection in the crystal structure of nanostructures of ZnS. The luminescence peaks around 485 nm in ZnS is attributed to sulfur vacancy related transition, whereas in literature, the peak around 490 nm is also being assigned as the transition of electrons from shallow donor level formed by interstitial Zn to shallow acceptor level formed by Zn vacancies. Similarly, the green emission from ZnS around 530 nm in our present experiment might be due to elemental sulfur species in these nanostructures. Thus, with increasing doses in these patterns, we see a reduction of point defects associated with such vacancy-impurity complexes. It is worthwhile pointing out that even though photoemission was observed in discs prepared with an electron dose of 25.2 mC/cm2 (Figure 6b), the

Table 2. Characteristic Infrared Absorption Peaks of Zinc Butylxanthate Resist absorption peak (cm−1)

assignment

peak number

1455 1381 1195 1112 1043

νas(−CH3) νs(−CH3) ν(C−O−C) ν(C−O) ν(CS)

1 2 3 4 5

changes are seen in the Raman spectrum at an electron dose of 25.2 mC/cm2, the corresponding FTIR spectrum, on the other hand, shows profound changes with complete disappearance of characteristic absorption peaks associated with zinc butylxanthate. It suggests that smaller dose is required to break the bonds in order for the resist to become insoluble in developers like chloroform. Unlike the Raman spectra that show the evolution of ZnS (Figure 5a), the corresponding FTIR spectra remain almost unchanged with the increasing electron dose except for the appearance of a weak band at 960 cm−1 most likely coming from ZnS (Figure 5b).25 In order to gain a better understanding of electron dosedependent microstructural changes and correlate them with the observations made in micro-Raman and micro-FTIR studies, high-resolution transmission electron microscopy (HR-TEM) was carried out on 1 μm size discs patterned on a 20 nm thick silicon nitride TEM membrane using the electron beam lithography machine at the same doses as studied in Figure 5. HR-TEM images show that, at lower electron doses, nanocrystals of ZnS of ∼2 nm appear, which grow in size when the dose is increased (Figure 6a). Furthermore, the selected area electron diffraction (SAED) pattern shows a diffuse ring at lower electron doses corresponding to the (111) plane of cubic ZnS, which becomes sharper and stronger in intensity with increasing patterning dose. Since the formation of ZnS happens due to the energy supplied by an electron beam at room temperature (assuming no heating effects due to electrons), it is expected that nanocrystals of ZnS will have a number of surface states due to 9924


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Figure 6. (a) High-resolution transmission electron microscopy images of structural evolution of ZnS nanocrystals with increasing electron dose. Notice the presence of lattice fringes of ZnS. The inset shows the SAED pattern for the corresponding TEM image. (b) Photoluminescence from 50 μm wide disks of ZnS patterned at various doses as indicated below each dot (in mC/cm2). Their corresponding maximum intensities are plotted to understand the change in photoemission with dose in a semiquantitative way. (c) Optical microscopy image of electron beam patterned motif (left) and the corresponding confocal microscopy image (right) showing the defect-induced photoluminescence from ZnS nanocrystals when imaged between 500 and 600 nm wavelength range (scale bar = 10 μm).

Chugaev elimination, giving only volatile byproducts.17,18,31,32 In the Chugaev elimination reaction, an alkene is formed by a concerted syn-elimination from a six-membered cyclic transition state.32 The TGA data (Figure 2a) for zinc butylxanthate gives interesting perception into its mode of decomposition. It is seen that zinc butylxanthate undergoes two clear stages in its thermal decomposition via the Chugaev elimination: from 110−180 °C, with almost complete decomposition of both xanthate ligands to leave Zn(HS)2 (observed: 34.9%; calculated: 36.1%) and finally a much slower decomposition between 250 and 350 °C to ZnS (observed: 27.4%; calculated: 26.8%). The closeness of theoretical and observed ceramic yields suggests almost 100% conversion to ZnS, indicating a clean thermal decomposition. Does a two-step decomposition of zinc butylxanthate involving the Chugaev elimination also happen under an electron beam? It seems to be so. In the first step, only a small amount of electron dose (25.2 mC/cm2) is

corresponding Raman spectrum showed no evidence of the presence ZnS (Figure 5a). This may be due to the fact that ZnS nanocrystals thus formed are too small and well below the sensitivity of Raman spectroscopic probing. When the dose is increased to 29.2 mC/cm2, the first tentative Raman signal of ZnS appears, while the corresponding photoluminescence gets stronger. Figure 6c shows optical microscopy and corresponding confocal microscopy images of a patterned motif using an electron beam at a dose of 59.2 mC/cm2. When imaged in the 500−600 nm wavelength range using a confocal microscope, the motif shows strong defect-induced photoluminescence due to the presence of ZnS nanocrystals. What is the possible electron beam damage mechanism of zinc butylxanthate that leads to the formation of ZnS nanocrystals? Before we answer this question, let us briefly digress to discuss the thermal decomposition. It has been suggested that metal xanthates thermally decompose via the 9925


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Figure 7. Tentative proposal of the Chugaev elimination reaction for breakdown of zinc butylxanthate resist under an electron beam to give ZnS.

required for the complete elimination of characteristic absorption peaks associated with zinc butylxanthate (Figure 5b). The corresponding Raman spectrum shows minimal changes. In the second step, with increasing an electron dose (≥34.2 mC/cm2), the first-order LO and second-order LO2 modes of ZnS appear at 344 and 691 cm−1, respectively (Figure 5a). Thus, exposure of zinc butylxanthate to a stream of electrons provides activation energy for the onset of the Chugaev elimination reaction (Figure 7). This reaction involves a concerted three electron pair shift that leads to the formation of olefin, release of OCS, and formation of Zn(HS)2. Further exposure to an electron beam leads to the conversion of Zn(HS)2 to ZnS and H2S; the latter and other volatile compounds are eliminated via the vacuum system of the lithography machine. The property of decomposition of zinc butylxanthate resist to give ZnS under an electron beam was exploited to make sub-10 nm features by exposing single pass lines and dots (Figure 7). Understandably, single pass features require considerably higher dose than writing larger feature sizes. Figure 8a,b shows 8 nm wide lines of ZnS in a grid of 200 nm pitch. However, when the pitch is reduced to 50 nm, it is possible to reduce the feature size to about 6 nm (Figure 8c,d). This is due to the fact that the denser grid provides a much better support to keep the lines erect, thus minimizing the dose requirement leading to finer lines. The aspect ratio of these lines is ∼4. Figure 8e,f shows 10 nm ZnS dots with pitches of 25 and 22 nm, respectively. A pitch of 20 nm shows merging of dots into each other. The ability to pattern such nanoscale features can be attributed to small molecular size and molecular weight of zinc butylxanthate resist. The ability to directly pattern ZnS using an electron beam offers several advantages over traditional methods of self-

assembly of sulfide nanoparticles. Our technique not only provides precise control over the positioning of patterns due to lack of lift-off and etching steps but also provides flexibility in generating diverse shapes and sizes. Furthermore, our technique also provides a potential way to place diverse metal sulfides precisely, whether juxtaposed or potentially, as a capping material over another.

CONCLUSIONS Zinc alkylxanthates prepared via the chemical reaction between zinc acetate and potassium alkylxanthates were studied for their suitability as electron beam resists. Although no significant trends in electron beam sensitivity and contrast were observed among them, zinc butylxanthate was chosen for further study due to its better solubility and development characteristics. Its time-resolved electron beam damage characteristics were investigated using micro-Raman and micro-FTIR spectroscopies. They show that exposure to a stream of electrons leads to quick disappearance of xanthate moieties most likely via the Chugaev elimination, and further increase of electron dose results in the appearance of ZnS, thereby making the exposed resist insoluble in developers such as chloroform. The presence of ZnS was confirmed by high-resolution transmission electron microscopy and SAED studies showing the appearance of lattice fringes and rings of cubic ZnS, respectively. The property of electron beam damage of zinc butylxanthate was successfully utilized to pattern ∼6 nm lines and 10 nm dots with pitch as close as 22 nm of ZnS. Nanocrystals of ZnS thus obtained were found to show defect-induced photoluminescence when studied using the confocal microscopy. Our method offers an easy and uncomplicated route to obtain complicated sulfide structures for potential optical applications. 9926


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Figure 8. Scanning electron microscopy images of (a,b) 8 nm and (c, d) 6 nm ZnS lines in an approximately 200 and 50 nm square grids, respectively; (e,f) 10 nm ZnS dots with pitches of 25 and 22 nm, respectively. general area detector diffraction system equipped with Cu Kα source was used. Raman spectroscopy was used to understand the evolution of phase at different heat-treatment conditions. Electron Beam Lithography. High-resolution electron beam nanolithography was carried out in an Elionix ELS 7000 operating at 100 kV with a probe current of 0.5 nA. Zinc alkylxanthate resists were spin-coated on silicon substrates. Exposed resists were developed in chloroform and rinsed in isopropyl alcohol for 10 s each. The samples were blown dry using a nitrogen gun. Characterization of Exposed Resists. A Veeco scanning probe microscope (SPM), operating in AFM tapping mode, was used for studying the heights of patterned zinc alkylxanthate samples exposed at different electron doses. Antimony-doped silicon tips (TESPA-V2) from Bruker with resonant frequency between 230 and 410 kHz were used. The raw images were flattened using Nanoscope Analysis 1.10 software provided with the Veeco SPM system. The pattern heights were then obtained using the step height option provided in the Nanoscope Analysis package. The micro-Raman measurements on thermally treated and electron beam exposed zinc butylxanthate (disc size = 50 μm) were performed using a Jobin-Yvon LabRAM HR system. A 325 nm He−Cd laser line was used as the UV excitation source. The laser power on the samples surface was kept very low to avoid laser heating, and spectra were acquired at a shorter acquisition time to avoid UV laser-induced degradation of ZnS patterns. A PerkinElmer AutoIMAGE FTIR microscope attached to PerkinElmer Spectrum 2000 FTIR was used for micro-FTIR of exposed resist. An Olympus FLUOVIEW F1000 V

MATERIALS AND METHODS Chemicals. Zinc acetate dihydrate (≥98%), zinc diethyldithiocarbamate (97%), anisole (anhydrous, 99.7%), and chloroform (≥99.5%) were purchase from Sigma-Aldrich (Singapore). Potassium ethylxanthate (>95%), potassium propylxanthate (>90%), potassium isopropylxanthate (>98%), potassium butylxanthate (>95%), and potassium pentylxanthate (>97%) were purchased from TCI Chemicals (Japan). All chemicals were used without further purification. Resist Preparation. Zinc acetate dihydrate and potassium alkylxanthates (molar ratio 1:2) were separately dissolved in deionized water and reacted by adding the former to the latter. A white precipitate of zinc alkylxanthate appeared immediately. However, the reaction was allowed to continue for another 10 min. The precipitate was filtered, washed five times with deionized water, and dried under vacuum overnight. Resists for electron beam patterning were prepared by dissolving 0.2 g of zinc alkylxanthate in 3 mL of chloroform and 0.5 mL of anisole for patterning thick structures. For fine patterning, the amount of solvents used was doubled. All the resist solutions were filtered before spincoating on substrates. Precleaned 1 cm × 2 cm silicon wafers with 50 nm thick Si3N4 were used as substrates. Characterization of Heat-Treated Resists. Thermogravimetric analysis and differential scanning calorimetry were performed in a nitrogen atmosphere (flow rate = 30 mL/min; heating rate = 10 °C/ min) using Netzsch STA 449 F1 Jupiter simultaneous thermal analyzer. For XRD analysis of heat-treated thin films, a Bruker D8 9927


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ACS Nano laser scanning confocal microscope (upright) was used to acquire images of the electron beam exposed zinc butylxanthate discs. These discs were excited using a 50 W diode laser (405 nm) through a 20× objective lens with a numerical aperture = 0.85. The images were analyzed using a MATLAB code that identifies each single pixel within the image with its corresponding intensity. This enabled us to measure and compare the emission intensity of each disc as a function of the electron beam exposure dose. Smaller discs using the same electron doses were patterned on a 20 nm thick silicon nitride TEM membrane (purchased from TEMwindows.com). A Philips CM300 field-emission gun-equipped transmission electron microscope operating at 300 kV was used to acquire high-resolution images and SAED patterns of the patterned discs. An Elionix ESM-9000 scanning electron microscope was used for acquiring high-resolution images of the exposed patterns.

(4) Fang, X.; Zhai, T.; Gautam, U. K.; Li, L.; Wu, L.; Bando, Y.; Golberg, D. ZnS Nanostructures: From Synthesis to Applications. Prog. Mater. Sci. 2011, 56, 175−287. (5) Gautam, U. K.; Fang, X. S.; Bando, Y.; Zhan, J. H.; Golberg, D. Synthesis, Structure, and Multiply Enhanced Field-Emission Properties of Branched ZnS Nanotube−In Nanowire Core−Shell Heterostructures. ACS Nano 2008, 2, 1015−1021. (6) Fang, X. S.; Bando, Y.; Shen, G. Z.; Ye, C. H.; Gautam, U. K.; Costa, P. M. F. J.; Zhi, C. Y.; Tang, C. C.; Golberg, D. Ultrafine ZnS Nanobelts as Field Emitters. Adv. Mater. 2007, 19, 2593−2596. (7) Moore, D.; Wang, Z. L. Growth of Anisotropic One-Dimensional ZnS Nanostructures. J. Mater. Chem. 2006, 16, 3898−3905. (8) Virkki, K.; Demir, S.; Lemmetyinen, H.; Tkachenko, N. V. Photoinduced Electron Transfer in CdSe/ZnS Quantum Dot− Fullerene Hybrids. J. Phys. Chem. C 2015, 119, 17561−17562. (9) Liu, K.; Zhao, N.; Kumacheva, E. Self-Assembly of Inorganic Nanorods. Chem. Soc. Rev. 2011, 40, 656−671. (10) Carbone, L.; Nobile, C.; De Giorgi, M.; Sala, F. D.; Morello, G.; Pompa, P.; Hytch, M.; Snoeck, E.; Fiore, A.; Franchini, I. R.; et al. Synthesis and Micrometer-Scale Assembly of Colloidal CdSe/CdS Nanorods Prepared by a Seeded Growth Approach. Nano Lett. 2007, 7, 2942−2950. (11) Talapin, D. V.; Shevchenko, E. V.; Murray, C. B.; Kornowski, A.; Förster, S.; Weller, H. CdSe and CdSe/CdS Nanorod Solids. J. Am. Chem. Soc. 2004, 126, 12984−12988. (12) Pietra, F.; Rabouw, F. T.; Evers, W. H.; Byelov, D. V.; Petukhov, A. V.; de Mello Donegá, C.; Vanmaekelbergh, D. Semiconductor Nanorod Self-Assembly at the Liquid/Air Interface Studied by in situ GISAXS and ex situ TEM. Nano Lett. 2012, 12, 5515−5523. (13) Abramson, J.; Palma, M.; Wind, S. J.; Hone, J. Quantum Dot Nanoarrays: Self-Assembly With Single-Particle Control and Resolution. Adv. Mater. 2012, 24, 2207−2211. (14) Persano, L.; Camposeo, A.; Di Benedetto, F.; Stabile, R.; Laera, A. M.; Piscopiello, E.; Tapfer, L.; Pisignano, D. CdS−Polymer Nanocomposites and Light-Emitting Fibers by in situ Electron-Beam Synthesis and Lithography. Adv. Mater. 2012, 24, 5320−5326. (15) Rath, T.; Padeste, C.; Vockenhuber, M.; Fradler, C.; Edler, M.; Reichmann, A.; Letofsky-Papst, I.; Hofer, F.; Ekinci, Y.; Griesser, T. Direct Extreme UV-Lithographic Conversion of Metal Xanthates into Nanostructured Metal Sulfide Layers for Hybrid Photovoltaics. J. Mater. Chem. A 2013, 1, 11135−11140. (16) Radha, B.; Kiruthika, S.; Kulkarni, G. U. Metal Anion−Alkyl Ammonium Complexes as Direct Write Precursors to Produce Nanopatterns of Metals, Nitrides, Oxides, Sulfides, and Alloys. J. Am. Chem. Soc. 2011, 133, 12706−12713. (17) Lewis, E. A.; McNaughter, P. D.; Yin, Z.; Chen, Y.; Brent, J. R.; Saah, S. A.; Raftery, J.; Awudza, J. A. M.; Malik, M. A.; O’Brien, P.; et al. In situ Synthesis of PbS Nanocrystals in Polymer Thin Films from Lead(II) Xanthate and Dithiocarbamate Complexes: Evidence for Size and Morphology Control. Chem. Mater. 2015, 27, 2127−2136. (18) Kociok-Köhn, G.; Molloy, K. C.; Sudlow, A. L. Molecular Routes to Cu2ZnSnS4: A Comparison of Approaches to Bulk and Thin-Film Materials. Can. J. Chem. 2014, 92, 514−524. (19) Nam, S.-W.; Rooks, M. J.; Yang, J. K. W.; Berggren, K. K.; Kim, H.-M.; Lee, M.-H.; Kim, K.-B.; Sim, J. H.; Yoon, D. Y. Contrast Enhancement Behavior of Hydrogen Silsesquioxane in a Salty Developer. J. Vac. Sci. Technol. B 2009, 27, 2635−2639. (20) Brafman, O.; Mitra, S. S. Raman Effect in Wurtzite-and ZincBlende-Type ZnS Single Crystals. Phys. Rev. 1968, 171, 931−934. (21) Nilsen, W. G. Raman Spectrum of Cubic ZnS. Phys. Rev. 1969, 182, 838−850. (22) Schneider, J.; Kirby, R. D. Raman Scattering From ZnS Polytypes. Phys. Rev. B 1972, 6, 1290−1294. (23) Little, L. H.; Poling, G. W.; Leja, J. Infrared Spectra of Xanthate Compounds. II. Assignment of Vibrational Frequencies. Can. J. Chem. 1961, 39, 745−754. (24) Perpiñ a ń , M. F. Preparation and Characterization of Dicarboniltriphenylphosphinedi(xanthate) Molybdenum(II) and Tungsten(II) complexes. J. Inorg. Nucl. Chem. 1981, 43, 181−184.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b03951. Figure S1 shows collapsed nanolines of ZnS, revealing their height, used to calculate the aspect ratio of the lines; the image was obtained at a lower dose than the ones shown in Figure 8a,b; Figure S2 depicts 10 nm ZnS dots with a pitch of 21 nm; some of these dots are merging into each other due to their close proximity (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Mohammad S. M. Saifullah: 0000-0002-3172-041X Ramakrishnan Ganesan: 0000-0003-4122-3174 Suresh Valiyaveettil: 0000-0001-6990-660X Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the A*STAR Nanoimprint Foundry (Project No. 1525300037). The authors thank Mr. Poh Chong Lim of Institute of Materials Research and Engineering (A*STAR) for his assistance in XRD. The help of Ms. Hannah Lim and Ms. Yi Hui Ng is gratefully acknowledged in the acquisition of confocal microscopy images. M.B.-K.K. and A.I.S. would like to thank the Institute of Materials Research and Engineering (A*STAR) and the National University of Singapore, respectively, for offering them summer internships. REFERENCES (1) Gao, M. R.; Xu, Y. F.; Jiang, J.; Yu, S. H. Nanostructured Metal Chalcogenides: Synthesis, Modification, and Applications in Energy Conversion and Storage Devices. Chem. Soc. Rev. 2013, 42, 2986− 3017. (2) Aldakov, D.; Lefrançois, A.; Reiss, P. Ternary and Quaternary Metal Chalcogenide Nanocrystals: Synthesis, Properties and Applications. J. Mater. Chem. C 2013, 1, 3756−3776. (3) Mehta, R. J.; Zhang, Y.; Karthik, C.; Singh, B.; Siegel, R. W.; Borca-Tasciuc, T.; Ramanath, G. A New Class of Doped Nanobulk High-Figure-of-Merit Thermoelectrics by Scalable Bottom-up Assembly. Nat. Mater. 2012, 11, 233−240. 9928


Article

ACS Nano (25) Lee, G.-J.; Anandan, S.; Masten, S. J.; Wu, J. J. Sonochemical Synthesis of Hollow Copper Doped Zinc Sulfide Nanostructures: Optical and Catalytic Properties for Visible Light Assisted Photosplitting of Water. Ind. Eng. Chem. Res. 2014, 53, 8766−8772. (26) Wang, X.; Shi, J.; Feng, Z.; Li, M.; Li, C. Visible Emission Characteristics From Different Defects of ZnS Nanocrystals. Phys. Chem. Chem. Phys. 2011, 13, 4715−4723. (27) Zhang, Y.; Lu, F.; Wang, Z.; Wang, H.; Kong, M.; Zhu, X.; Zhang, L. ZnS Nanoparticle-Assisted Synthesis and Optical Properties of ZnS Nanotowers. Cryst. Growth Des. 2007, 7, 1459−1462. (28) Zhang, Z.; Wang, J.; Yuan, H.; Gao, Y.; Liu, D.; Song, L.; Xiang, Y.; Zhao, X.; Liu, L.; Luo, S.; et al. Low-Temperature Growth and Photoluminescence Property of ZnS Nanoribbons. J. Phys. Chem. B 2005, 109, 18352−18355. (29) Osman, M. A.; Othman, A. A.; El-Said, W. A.; Abd-Elrahim, A. G.; Abu-Sehly, A. A. Thermal Annealing and UV Irradiation Effects on Structure, Morphology, Photoluminescence and Optical Absorption Spectra of EDTA-capped ZnS Nanoparticles. J. Phys. D: Appl. Phys. 2016, 49, 055304. (30) Li, Y.; Zhang, L.; Kisslinger, K.; Wu, Y. Green Phosphorescence of Zinc Sulfide Optical Ceramics. Opt. Mater. Express 2014, 4, 1140− 1150. (31) Akhtar, J.; Afzaal, M.; Vincent, M. A.; Burton, N. A.; Hillier, I. H.; O’Brien, P. Low Temperature CVD Growth of PbS Films on Plastic Substrates. Chem. Commun. 2011, 47, 1991−1993. (32) Pradhan, N.; Katz, B.; Efrima, S. Synthesis of High-Quality Metal Sulfide Nanoparticles from Alkyl Xanthate Single Precursors in Alkylamine Solvents. J. Phys. Chem. B 2003, 107, 13843−13854.

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Electron Beam Lithography - ACS Publications - American Chemical

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