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Novel Surface Molecular Functionalization Route to Enhance Environmental Stability of Tellurium Containing 2D Layers Sijie Yang, Ying Qin, Bin Chen, V. Ongun Özçelik, Claire E. White, Yuxia Shen, Shengxue Yang, and Sefaattin Tongay ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14873 • Publication Date (Web): 01 Dec 2017 Downloaded from http://pubs.acs.org on December 5, 2017

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ACS Applied Materials & Interfaces

Novel Surface Molecular Functionalization Route to Enhance Environmental Stability of Tellurium Containing 2D Layers Sijie Yang1, Ying Qin1, Bin Chen1, V. Ongun Özçelik2,3, Claire E. White2,3, Yuxia Shen1, Shengxue Yang4, and Sefaattin Tongay1* 1

School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona 85287, United States 2

Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey 08544, United States

3

Andlinger Center for Energy and the Environment, Princeton University, Princeton, New Jersey 08544, USA

4

School of Materials Science and Engineering, Beihang University, Beijing 100191, People’s Republic of China Abstract: Recent studies have shown that tellurium based 2D crystals undergo dramatic structural, physical, and chemical changes under ambient conditions, which adversely impact their much desired properties. Here, we introduce diazonium molecule functionalization based surface engineering route that greatly enhances their environmental stability without sacrificing their much desired properties. Spectroscopy and microscopy results show that diazonium groups significantly slow down the surface reactions, and consequently gallium telluride (GaTe), zirconium telluride (ZrTe3) and molybdenum ditelluride (MoTe2) gain strong resistance to surface transformation in air or when immersed under water. Density functional theory calculations show functionalizing molecules reduces surface reactivity of Te-containing 2D surfaces by chemical binding followed by electron withdrawal process. While pristine surfaces structurally decompose due to strong reactivity of Te surface atoms, passivated functionalized surfaces retain their structural anisotropy, optical band gap, and emission characteristics as evidenced by our conductive AFM, PL and absorption spectroscopy measurements. Overall, our findings offer an effective method to increase the stability of these environmentally sensitive materials without impacting much of their physical properties. *

Corresponding author: [email protected]

Keywords: 2D materials, environmental stability, degradation, chemical functionalization, spectroscopy Introduction Atomically thin materials are a new class of materials where the atoms are confined in two dimensions in different crystalline symmetries1. Because of their extreme thinness, these materials largely benefit from novel material properties in the quantum confinement limit, but their large surface to volume ratio also makes them extremely sensitive to environmental effects2-5. While most two-dimensional (2D) materials, such as graphene, MoS2, and others, are known to be relatively stable, the environmental stability of other 2D materials is subject to question: Monoatomic 2D layers including black phosphorus (BPs)3, 6, silicene, antimonene, and germene5 all structurally deteriorate upon exposure to air due to their high reactivity to O2, H2O, CO2, and other gases (Figure 1a). Recent studies by various teams including ours have raised concerns over environmental stability of Tecontaining 2D layers such as anisotropic GaTe7-9, ZrTe310, and MoTe22. While these tellurium based 2D 1 ACS Paragon Plus Environment

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layers offer unique properties, i.e., infrared range band gaps, high structural, electrical, and optical anisotropy, their instability presents increased difficulties in their manufacturing and applications11-12. Studies to date show that air/surface interactions induce anisotropic to isotropic transition9, phase transition13, oxidization, and intercalation14 (Figure 1b). To overcome these challenges, it is necessary to develop cheap, reproducible, and manufacturing compatible post-processing techniques to extend the lifetime of these 2D crystals without causing any adverse effects on their physical properties. In this work, we have adapted surface functionalization chemistry route to protect tellurium based 2D crystals (Figure 1c) against detrimental effects of gaseous interactions at the 2D surfaces. In particular, diazonium based surface functionalization is carried out owing to its success in doping graphene based 2D material systems15-16. Our systematic studies show that when 2D surfaces are functionalized by -NO2 and -OCH3 substituted diazonium molecules (Figure 1d), 2D crystals possess much enhanced stability in air and even under water (Figure S1) which is known have adverse effects on these materials. Environmental Raman spectroscopy, PL spectroscopy, atomic force microscopy (AFM), absorption spectroscopy, and conductive AFM measurements confirm that diazonium surface functionalization enhances the surface stability without impacting its physical properties. Our results show that anisotropic response is retained, band gap stays unchanged, light emission efficiency remain largely unaffected.

Figure 1 a. Degradation scheme of tellurium based 2D material b. Raman spectrum acquired from exfoliated (orange) and aged (blue) GaTe demonstrates that aging process induces large surface transformation on the surface c. Side view of single layer monoclinic m-GaTe, 2H-MoTe2 and m-ZrTe3, and d. molecular structure of 4-nitro/methoxy benzenediazonium salts.

Sheets of tellurium containing materials, GaTe, ZrTe3, and MoTe2, were mechanically exfoliated from bulk crystals synthesized by chemical vapor transport (CVT) grown crystals onto 285 nm thermal oxide (SiO2) on Si substrates. Surface functionalization was carried out by immersing exfoliated tellurium based 2D crystals into 4-nitrobenzene diazonium solution for 15 minutes. Diazonium solution was prepared by dissolving 4-nitrobenzenediazonium tetrafluoroboride (C6H4N3O2·BF4) in acetonitrile (CH3CN) with 0.1M tetrabutylammonium hexafluorophosphate ((C4H9)4N·PF6) as electrolyte to make a 0.01M passivation solution. Afterwards the sample was rinsed with pure acetonitrile several times and dried 2 ACS Paragon Plus Environment

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under nitrogen flow. We note that successful surface diazonium functionalization largely depends on the concentration of the solution and exposure (duration) time to diazonium solution. For example, dilute diazonium solution is intuitively expected to have very little effect on their surface characteristics due to low surface coverage. In fact, we noticed a few unsuccessful passivation processes wherein the concentration of diazonium salt was kept below 10 mM. Meanwhile, diazonium concentration between 20 and 100 mM and treatment time ~15 minutes proved to be very effective without affecting the properties of the material. In our work, ideal functionalization conditions were determined to be 20 mM diazonium concentration and 15 minutes for passivation. Exfoliated 2D layers were first characterized through AFM, c-AFM, Raman, angle-resolved Raman, PL, and micro-absorption spectroscopy techniques to identify the surface quality/characteristics, as well as material properties prior to material degradation in air. Effects of air/surface interaction were investigated by in-situ measurements in air within a time frame, usually ~2 weeks, sufficient to observe environmental effects on 2D surfaces. During this aging process, two broad Raman peaks associated with amorphous phase TeOx emerge and eventually overtake intrinsic Raman features (Fig.2). Previously, our team has successfully shown that this surface effect is related to GaTe TeOx (x=2-3) + Ga2O3 transformation due to high reactivity between Te atoms in monoclinic GaTe and humidity (H2O), and similar oxidization effects have also been observed for ZrTe3 and MoTe2 as shown in Fig.2b9.

Figure 2 Effects of passivation on GaTe, ZrTe3 and MoTe2. Raman spectra acquired from as cleaved, aged, passivated, and passivated/aged a. GaTe b-c. ZrTe3 d-e. MoTe2

To prevent tellurium based materials from oxidization and aging effects, we have adopted diazonium molecule based surface functionalization post-processing method15-16. As outlined earlier, functionalization with diazonium molecules at optimized molar solution and exposure timeframes (see methods) was found to be highly effective in improving the material stability in air as well as under extreme condition (under water). As shown in Fig. 2a, 2c, and 2e, Raman peaks of functionalized Tecontaining materials remain unchanged after 2 weeks (our measurements run up to 4 weeks) for GaTe, ZrTe3, MoTe2, respectively. These findings were observed on more than 50 different samples with high accuracy and repeatability: over 95% of these samples were successfully passivated with our optimized surface functionalization condition. We note that this method was also found to be effective in enhancing 3 ACS Paragon Plus Environment


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material stability under extreme conditions: In a typical experiment, both as cleaved and passivated GaTe samples were immersed into deionized water for 5 hours. While the as cleaved sample was highly transformed during this process as evidenced by Raman spectra suggesting material instability under water or humidity -which is a significant roadblock for water splitting reactions-, the passivated sample showed only minimum changes in Raman spectrum (See Figure S1). In addition to 4nitrobenzenediazonium tetrafluoroboride, we also investigated another diazonium salt, 4methoxybenzenediazonium tetrafluoroboride, and results closely match to our findings on 4nitrobenzenediazonium tetrafluoroboride. As such, two investigated diazonium salts function and perform similarly to extend the stability of GaTe without any significant difference between two chemicals. After demonstrating improved stability of GaTe, ZrTe3, and MoTe2, we focus our attention to GaTe in particular to unravel the effect of diazonium functionalization on the physical characteristics of the GaTe surfaces. To understand the effect of surface functionalization on the surface morphology of 2D surfaces, we have performed careful AFM studies on pristine, aged, and functionalized surfaces (Figure 3). AFM measurements on pristine GaTe surface shows that upon exposure to air, the material thickness increases by 4.6 nm and the surface roughness increases by about 25% from 0.39 nm to 0.51 nm (Fig.3a-c). The increase in material thickness can be attributed to surface amorphization (crystalline GaTe to amorphous TeOx surface transformation) with greater bond lengths and the degree of bonding variation. In contrast, the functionalization of 2D surface increases the material thickness by diazonium molecule thickness (~12 nm), and interestingly the surface roughness remains unchanged (RMS functionalized vs aged: 0.36 vs 0.37 nm) after prolong exposure to air (Fig.3c-f). The increase in material thickness by surface functionalization can be attributed to molecular decoration of aryl salt groups on the surface. Since the surface roughness remains largely unchanged after the functionalization process (unlike environmental deterioration), aryl group modification on the surface is likely retain the overall crystallinity of 2D surfaces. Environmental oxidization process, however, leads to amorphization and increased roughness due to loss of high crystallinity. We note that the passivated sample gains another ~1 nm in thickness during aging process, and the origin of this effect remains unclear and warrants further studies.

Figure 3 Surface morphology and thickness changes of GaTe during environmental aging AFM images taken from a. pristine, b. aged GaTe c. RMS roughness data extracted out from different sample sets with schematic representation of the

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surfaces. d. pristine, e. diazonium decorated, and f. aged functionalized GaTe nanomaterials. [Inset] Inset images depict surface transformation effects.

To understand the effects of diazonium functionalization on the material performance, we have performed PL and Raman spectroscopy measurements. PL data collected from pristine (red) and functionalized (yellow and green solid lines in Figure 4a) reveal that PL intensity shows ~50-80% variation while the PL peak shape and position remain unchanged. In Figure 4a, the PL spectrum was displayed between 700780nm range since m-GaTe displays one single band-to-band radiative recombination PL peak at 740 nm, and no other PL peaks are observed outside 700-780 nm range. Similar PL peak shapes and positions suggest that the optical band gap (quasi-particle gap) of the material is insensitive to surface functionalization on the surface. However, observed reduction in PL emission intensity is likely associated with changes in radiative to non-radiative recombination timescales. As theoretically and experimentally demonstrated later in the article, diazonium molecules withdraws electrons from host matrix (GaTe or other pseudo-1D materials) and induces hole (p-type) doping effect. Increased hole concentration, in return, stabilizes Auger non-radiative recombination processes wherein the increased carrier density enhances non-radiative losses to charges that are not involved in the radiative recombination processes. More specifically, photons generated by recombination of photo-excited electrons and holes transfer their energy non-radiatively to free-carriers (in this case holes induced through surface charge transfer mechanisms) instead of generating radiative emission (photoluminescence). Here, we note that the environmental surface deterioration on GaTe results in loss of much desired anisotropic material response as shown in Figure 4d. To understand the effect of changes on the 2D surface on the anisotropic response, we have performed angle resolved Raman spectroscopy (ARS). In this method, material is excited under linearly polarized laser and the polarization vector is scanned with respect to laboratory anisotropy (b-axis) axis (θ) as shown in Figure 4b-c. Previously, the structural anisotropy of GaTe has been observed and correlated by angle resolved Raman spectroscopy (ARS)8, 14, 17: it has been established that one of the prominent Raman peaks at 177 cm-1 correlates to atomic vibrations along b axis. In accord with these findings, the Raman intensity of 177cm-1 peak (of pristine GaTe -red solid line-) is maximized when the polarization vector of the laser excitation runs parallel to b axis (red circles and solid line in Fig.4d). Interestingly, after aging in air for two weeks, Raman polar plots appear circular (Figure 4d blue line) due to anisotropic to isotropic transition induced by the surface transformation (oxidization) process9. Diazonium surface functionalization, on the other hand, inhibits this anisotropic to isotropic transition by preventing immediate gas-surface interaction and functionalized GaTe surfaces remain highly anisotropic shown in Figure 4e. This suggests that diazonium molecular decoration on the surface prevents surface oxidization effects and effectively prevents anisotropic to isotropic cross-over which has been one of the difficulties in the field for their applications in nanoelectronics and photonics.



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Figure 4 Effect of surface functionalization on physical properties of GaTe a. Photoluminescence spectra of as cleaved (red line), passivated (yellow line) and aged passivated (green line) GaTe b-c. Schematic description of angle resolved Raman spectroscopy, and definition of polarization angle θ with respect to anisotropy axis (b) and polarization vector (P) c. side and top view of anisotropic monoclinic phase GaTe. Here, peak at 177 cm-1 involves atomic vibrations predominantly along the chain (anisotropy) direction along b-axis. d. angle resolved Raman spectra of 177cm-1 peak for freshly exfoliated GaTe (red) and aged GaTe (blue). e. Similarly, polar plots for 177 cm-1 peak collected on functionalized (green) and aged functionalized (yellow) GaTe sheets.

Surface modification by adsorbed molecules have been reported to have substantial effects on the properties of various 2D materials as result of charge transfer18. The electron extraction or injection of target material depends on the relative electronegativity between the material and dopant molecules. In our case, the diazonium is expected to be p-type dopant (electron extraction) because of its carbon and nitrogen elements6, 19. To further understand the charge transfer characteristics at diazonium/GaTe interface, we have performed careful surface potential measurements through Kelvin probe force microscopy (KPFM). Surface potential measurements were performed by isolating GaTe nanomaterials onto conductive indium tin oxide (ITO) substrates, and potential measurements were carried out before and after passivation. Result shown in Figure 5a demonstrates that the surface potential of sample increases from 4.46 eV to 4.56 eV, i.e. the Fermi level of GaTe decreases by 100 meV as depicted in the band diagram shown in Figure 5b. Our conductive AFM measurements (Fig.5d) on GaTe/ITO junctions also demonstrate that the IV characteristics remain largely unchanged except the overall current (current density) decreases. This can simply be attributed to the presence of atomic layer coating of diazonium molecules on GaTe surface resulting in a tunnel barrier effectively decreasing the overall current reading. Indeed, this effect can clearly be observed from our density functional theory (DFT) simulations where diazonium molecules were found to withdraw electrons from chalcogen tellurium sites. Our computational studies show that the diazonium molecule interacts strongly with GaTe through two different routes: Firstly, diazonium group of the diazonium molecule binds from the Ga-Ga bond at the bridge site (see Fig. 5c) with a binding energy of 480 meV. Alternatively, when diazonium molecule is close enough to the Te site, N2 dissociates and C atom in the benzene ring anchors to Te site with a binding energy of 0.55 eV as shown in Figure5c. In this configuration, a Te-C bond is established and electronic charge transfers from Te to C. The isosurfaces of the total charge density is shown in the lower panel of Figure 5c. Our Mulliken analyses show that the net charge transferred from a Te atom to an adjacent C atom is 0.46 electrons per bond. The interaction between monolayer GaTe and diazonium also manifests itself in the electronic densities of states as presented in Fig.5. After this interaction, new electronic states (localized states) appear in close proximity to valance band which makes the overall 6 ACS Paragon Plus Environment

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system a p-type semiconductor. It is also noteworthy to mention that it is the binding at the tellurium site that passivates highly reactive Te atoms and leads to enhanced material stability.

Figure 5. Electronic effects of diazonium functionalization a. Surface potential map of GaTe before (left) and after (right) diazonium passivation. b. Band diagram of GaTe before/after passivation. c. DFT model and charge density map of passivated GaTe. d. I-V curve of as cleaved (black line) and passivated (red line) GaTe.

Conclusion In summary, our results show that environmental stability of Te-containing 2D sheets are of particular concern due to high chemical reactivity of Te atoms but our studies suggest that diazonium based surface functionalization can be utilized to increase the stability (shelf-life) of GaTe, ZrTe3, and MoTe2 surfaces without altering their much desired properties. Diazonium based post-processing method is simple, costeffective, scalable, and reproducible with (~95% success rate). After the diazonium functionalization, 2D sheets demonstrate much enhanced stability against gases commonly found in air such as O2, humidity H2O(g), N2, and others. as well as perform well under water (H2O(l)). Functionalization process does not lead to band renormalization, and their physical and chemical properties remain largely unchanged. However, surface functionalization leads to a substantial amount of charge transfer which leads to different chemical potentials, carrier density, and exciton recombination timescales. Reported chemical routes and post-processing techniques helps to overcome one of the biggest challenges, material stability, of 2D Te-containing layers and enables their integration into applications in electronics, photonics, and energy conversion fields. Acknowledgements: S.T acknowledges support from NSF DMR-1552220 and NSF CMMI-1561839. V.O.O and C.E.W acknowledge funding from the Princeton Center for Complex Materials, a MRSEC supported by NSF Grant DMR 1420541. The calculations presented in this article were performed on computational resources supported by the Princeton Institute for Computational Science and Engineering (PICSciE) and the Office of Information Technology's High Performance Computing Center and Visualization Laboratory at Princeton University.

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Sample preparation: GaTe, ZrTe3 and MoTe2 were exfoliated from bulk crystals onto Si wafer coated with 285 nm SiO2 layer by standard mechanical exfoliation with Scotch tape. All exfoliated samples were immediately used for measurements unless otherwise stated. All the layered materials were synthesized through either chemical vapor transport or flux zone growth technique with the exception of GaTe crystals (Bridgman synthesis). Surface passivation: 4-nitrobenzenediazonium tetrafluoroboride was dissolved in acetonitrile with 0.1M tetrabutylammonium hexafluorophosphate as electrolyte to make a 0.01M passivation solution. In a typical experiment, exfoliated sample on SiO2/Si was immersed in 1 mL of passivation solution for 15 minutes. Afterwards the sample was rinsed with pure acetonitrile several times and dried under nitrogen flow. Raman and photoluminescence spectroscopy: Raman spectra were recorded on Renishaw in via Raman microscope with a 488 nm excitation laser in the backscattering configuration with 2400/mm grating. The spot size of focused laser was ~1 µm and the laser power was 37.5 µW. All measurements were performed under ambient conditions at room temperature unless otherwise stated. Raman spectra of ZrTe3 samples were recorded in a vacuum chamber to avoid the rapid laser induced surface transformation. Angle resolved Raman measurements were performed on the same Raman microscope with an additional rotation sample stage. A linear polarized laser was used as light source and a normal configuration between the laser source and detector polarizer was used. Photoluminescence spectra were recorded on the same Raman microscope system with 1200/mm grating. Atomic force microscopy: Topography and electrical measurements were both conducted by Bruker Multimode 8 with Nanoscope V controller. Samples are mounted on ITO substrate for electrical measurements. Silver or nickel paste was used to connect the ITO surface to sample mounting disk. Surface potential was measured in amplitude modulation mode with Au-coated probe. Before each GaTe measurement, the work function of tip was calibrated by scanning the freshly exfoliated HOPG. PtIr coated tips were used for conductive AFM measurement in contact mode. The scanning size was 256 x 256 and the scanning speed was set to 1Hz. Data was analyzed by Gwyddion software and MATLAB. DFT calculations: DFT calculations were performed within generalized gradient approximation (GGA) including van der Waals corrections20. Projector-augmented wave potentials PAW21 were used and the exchange-correlation potential was approximated with Perdew-Burke-Ernzerhof, PBE functional22. In the geometry optimizations, the Brillouin zone was sampled by (5x5x3) k-points in the Monkhorst-Pack scheme where the convergence in energy as a function of the number of k-points was tested. The energy convergence value between two consecutive steps was chosen as 10-5 eV. A maximum force of 0.05 eV/A was allowed on each atom. Optimum geometries of the structures, charge densities and electronic densities of states were calculated using the VASP23 software. The Mulliken charges were computed using the SIESTA code24 where a 200 Ry mesh cutoff was chosen and core states were replaced by norm conserving, nonlocal Trouiller-Martins pseudopotentials25. Dipole corrections26 were applied in order to remove spurious dipole interactions.

Supporting information: Effect of diazonium functionalization on GaTe degradation in water, as evidenced by Raman spectroscopy (Figure S1)


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