Molybdenum Carbamate Nanosheets as a New Class of Potential

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Molybdenum carbamate nanosheets as a new class of potential phase change materials Maksym Zhukovskyi, Vladimir V. Plashnitsa, Nattasamon Petchsang, Anthony Ruth, Anshumaan Bajpai, Felix Vietmeyer, Yuanxing Wang, Michael Brennan, Yunsong Pang, Kalpani Werellapatha, Bruce A Bunker, Soma Chattopadhyay, Tengfei Luo, Boldizsar Janko, Patrick Fay, and Masaru Kuno Nano Lett., Just Accepted Manuscript • Publication Date (Web): 16 May 2017 Downloaded from http://pubs.acs.org on May 16, 2017

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Molybdenum carbamate nanosheets as a new class of potential phase change materials Maksym Zhukovskyia*, Vladimir Plashnitsaa, Nattasamon Petchsangb, Anthony Ruthc, Anshumaan Bajpaia, Felix Vietmeyera, Yuanxing Wanga, Michael Brennana, Yunsong Pangd, Kalpani Werellapathac, Bruce Bunkerc, Soma Chattopadhyaye†, Tengfei Luod, Boldizsar Jankoc, Patrick Fayf, Masaru Kunoa* a

Department of Chemistry and Biochemistry, University of Notre Dame, 251 Nieuwland Science Hall, Notre Dame, IN 46556, United States

b

Department of Materials Science, Faculty of Science, Kasetsart University, 50 Ngam Wong Wan Road, Lat Yao Chatuchak, Bangkok 10900, Thailand

c

Department of Physics, University of Notre Dame, 225 Nieuwland Science Hall, Notre Dame, IN 46556, United States

d

Department of Aerospace and Mechanical Engineering, University of Notre Dame, 371 Fitzpatrick Hall, Notre Dame, IN 46556, United States e

Advanced Photon Source, Argonne National Laboratory, 9700 S. Cass Avenue, Building 401, Lemont, IL 60439, United States

ACS Paragon Plus Environment

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Department of Electrical Engineering, University of Notre Dame, 275 Fitzpatrick Hall, Notre Dame, IN 46556, United States

KEYWORDS: amorphous • molybdenum • carbamate • variable-range hopping • phase transition • two-dimensional

ABSTRACT:

We report, for the first time, the synthesis of large, free-standing, Mo2O2(µ-S)2(Et2dtc)2

(MoDTC) nanosheets (NSs), which exhibit an electron-beam induced crystalline-to-amorphous phase transition. Both electron beam ionization and femtosecond (fs) optical excitation induce the phase transition, which is size-, morphology- and composition-preserving. Resulting NSs are the largest, free standing regularly-shaped two-dimensional amorphous nanostructures made to date. More importantly, amorphization is accompanied by dramatic changes to the NS electrical and optical response wherein resulting amorphous species exhibit room temperature conductivities 5 orders of magnitude larger than those of their crystalline counterparts. This enhancement likely stems from the amorphization-induced formation of sulfur vacancy-related defects and is supported by temperature-dependent transport measurements, which reveal efficient variable range hopping. MoDTC NSs represent one instance of a broader class of transition metal carbamates likely having applications because of their intriguing electrical properties as well as demonstrated ability to toggle metal oxidation states.

Phase-change materials are of significant interest due to their application in rewritable optical data storage1 and in non-volatile electronic memories.2 They are also strong candidates for thermoelectric


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applications.3 The usefulness of phase change materials stems from the existence of distinct amorphous and crystalline phases, which exhibit pronounced differences in their electrical and optical response. These differences arise from corresponding variations in bonding and atomic coordination number.4,5 In general, amorphous phases are highly resistive, have conductivities several orders of magnitude lower than those of their crystalline counterparts, and posses larger band gaps.1,2 Amorphous materials have simultaneously gained interest within the context of catalytic,6 photovoltaic,7 battery8 and electronic9 applications. This stems, in large part, from the absence of grain boundaries, the existence of high active site densities and the ease of substitutional doping.6,10,11 Additionally, amorphous low dimensional materials are of special interest given their large surfaceto-volume ratios and their size-tunable optical, electrical, and charge transport properties.12,13,14,15 Despite the many synthetic approaches now available for achieving dimensional control over crystalline nanostructures, corresponding morphological control over amorphous nanostructures remains lacking. This is because surface energy minimization as well as the absence of long range atomic order favors the formation of energy minimizing morphologies.16,17,18 Consequently, only a few publications describe the synthesis and electrochemical properties of amorphous twodimensional (2D) materials.19,20,21 We report, for the first time, the synthesis of large, free-standing, Mo2O2(µ-S)2(Et2dtc)2 (MoDTC, DTC=dithiocarbamate) nanosheets (NSs), which exhibit an electron-beam induced crystalline-toamorphous phase transition. The NS composition, size and morphology are all retained during the phase transformation. More importantly, amorphization is accompanied by dramatic changes to the NS electrical and optical response wherein resulting amorphous species exhibit room temperature conductivities 5 orders of magnitude larger than those of their crystalline counterparts.


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Complementary temperature-dependent current-voltage (I-V) measurements suggest that this dramatic conductivity improvement stems from the creation of defect states which lead to efficient variable range hopping (VRH). Of note, MoDTC is one example of broader class of transition metal carbamates with potential applications due to prior demonstrations illustrating controlled toggling of the metal oxidation state.22

Crystalline MoDTC NSs Crystalline MoDTC NSs are made through the thermolysis of a single source precursor [Mo(Et2CNS2)4] in diphenyl ether (Ph2O). Resulting NSs possess an average length and width of 12.84 (±3.90) µm and 0.91 (±0.14) µm (sample size = 33) [Figure 1a]. NS thicknesses are independently established using atomic force microscopy (AFM) [average thickness 460 (±30) nm (sample size = 19)] (Figure 1b).

Details regarding the single source precursor/MoDTC NS

synthesis, single source precursor mass spectra, and additional MoDTC NS TEM images/sizing histograms can be found in the SI as well as in Figures S1–S3). Associated selected area electron diffraction (SAED) patterns (Figure 2a) confirm that the asproduced sheets are crystalline. Observed diffraction spots readily index to the (020), (20-4) and (224) planes of C2/c MoDTC with corresponding d-spacings of 0.35 nm, 0.41 nm and 0.55 nm. The SAED results suggest that MoDTC NSs grow along with {100} planes oriented normal to the electron beam. The combination of independent XRD (Figure S4) and SAED data allows us to assign MoDTC NS growth directions and growth facets as shown in Figure S5. A more detailed discussion about the XRD structure assignment can be found in the SI.


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Figure 1. (a) Low-magnification TEM image of MoDTC NSs. (b) AFM topography and linescan (inset) of an individual MoDTC NS. MoDTC NS structural assignment is further corroborated though independent Extended X-ray Absorption Fine Structure (EXAFS) analyses, which yield Mo-S, Mo-Mo, and Mo-O bond lengths in excellent agreement with model predictions [Mo-S: 2.38±0.01 Å, predicted: 2.383 Å; Mo-Mo: 2.83±0.01 Å, predicted: 2.810 Å; Mo-O: 1.67±0.02 Å, predicted: 1.67 Å]. EXAFS fits furthermore suggest Mo oxidation states greater than 4+. Detailed EXAFS analyses can be found in the SI (Figure S6).

Amorphous MoDTC NSs Crystalline MoDTC NSs readily undergo a crystalline-to-amorphous phase transition under electron beam irradiation. As seen from TEM images in Figure 2a (top versus bottom), continuous exposure of the NSs to an electron beam leads to the disappearance of crystalline bend contours. Amorphization is most evident through corresponding SAED patterns, which disappear rapidly under continuous electron beam exposure (200 kV, associated beam current density: 1.6×10-4 A/cm2). Additional SAED patterns, revealing MoDTC NS amorphization, can be found in Figure S7. Corresponding contrast differences emerge in scattered light images of the NSs. Figure S8 shows an example where electron beam irradiation has been used to selectively amorphize sections of


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individual NSs. Analogous amorphization-induced contrast differences in NS scattered light images have been acquired using 387 nm fs pulsed excitation (Figure S9). These contrast differences stem from changes to the optical response of the sheets as seen through variations in the linear absorption. Figure 2b shows before/after differences in ensemble absorption spectra where it is apparent that a broad band, centered at 750 nm, emerges following conversion. These changes are assigned to irradiation-induced defect state formation as discussed below. This contrasts to the response of conventional phase change materials where significant band gap increases occur following amorphization.5 Note that preliminary studies suggest that light-induced recrystallization, a critical requirement for any eventual application is possible. However, significant additional efforts are needed to conclusively verify this. In all cases, NS amorphization is corroborated by loss of characteristic Raman features. Figure S8 compares Raman spectra taken from crystalline and amorphous sections of the same NS. The lack of distinct Raman resonances likely stems from significant line broadening and/or intensity decreases following amorphization.23,24 Of notable interest is that the composition, size and morphology of MoDTC NSs are all retained during the crystalline-to-amorphous phase transition. What results are NSs which, to best of our knowledge, are the largest, regularly-shaped two dimensional amorphous nanostructures made to date. Gross compositional conservation and retention of Mo’s 5+/6+ oxidation states are evident in X-ray photoelectron spectroscopy (XPS) measurements (Figure S10), which show identical peak positions and peak areas before and after amorphization.

Amorphization mechanism MoDTC electron beam-induced ionization appears to be the dominant origin of the observed phase transition.25 This assessment is based on transformations which occur under low electron


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energies26 and through amorphization time constants (τ) which increase linearly (from 0.37 to 2.00 s) with increasing incident electron energies (from 120 to 200 keV, constant beam current density of IBeam = 3.1×10-5 A/cm2) (Figure 2c). SAED image series, such as that in Figure S7, illustrate the metastable electron diffraction at a given beam energy. The trend is characteristic of ionization damage and stems from decreasing specimen ionization cross sections with increasing electron energies.27,28 TEM-induced specimen heating has also been investigated as a possible cause of NS amorphization. This has been probed by monitoring beam current-induced changes to amorphization τ-values given the proportionality between IBeam and the induced specimen temperature (Equation

S1, SI). Figure 2d shows that increasing IBeam from 3.1×10-5 A/cm2 to 1.6×10-4 A/cm2 decreases τvalues from 6.23 to 0.51 s. The trend is consistent with what is expected from an activated process.

Figure 2. (a) False color TEM images and SAED of MoDTC NSs before and after amorphization. (b) Ensemble absorption spectra of MoDTC NSs before (solid black line) and after (dashed red line) fs optical irradiation. Amorphization time constant from (c) electron beam energy and (d) current density measurements.


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Separate ensemble thermal transport measurements were therefore conducted to yield estimates of the crystalline NS thermal conductivity (κ). An obtained lower limit is κ=0.08 Wm-1K-1. Together with relevant IBeam–values (between 3.1×10-5 A/cm2 and 1.6×10-4 A/cm2, constant accelerating voltage of 200 keV), we estimate a maximum temperature rise of ∆T~10-4–10-3 K for the specimen in Figure 2d. TEM sample heating is therefore discounted as the origin of MoDTC amorphization. Additional details regarding these thermal transport measurements and specimen heating estimates can be found in the SI.

Amorphization-enhanced conductivities Using single MoDTC NS transport measurements, we have established that dramatic conductivity changes occur upon amorphization. Specifically, a conductivity enhancement exceeding 5 orders of magnitude is observed following the phase transition. The origin of this enhancement likely stems from an amorphization-induced increase in the density of states available for hopping transport, as detailed below. I–V measurements were performed on single MoDTC NS using two electrode devices (Figure 3a). Figure 3b shows a typical room temperature I–V response of a crystalline NS. Crystalline NS devices exhibit non-ohmic behavior. They additionally show hysteresis loops under forward and reverse bias. At Vbias=2 V the estimated NS conductivity is σ=1.13×10-7 Ω-1m-1. The observed hysteretic behavior likely stems from charge trapping/detrapping into crystalline NS defect states. Existence of trapping and detrapping processes is confirmed by the presence of a displacement current at 0 V, which increases with increasing sweep voltage. A more detailed discussion about the preparation of NS devices as well as trap state involvement in their hysteretic behavior can be found in the SI as well as in Figure S11.


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In dramatic contrast, amorphous MoDTC NS devices exhibit an ohmic response as well as a substantial 5 orders of magnitude increase in conductivity (Figure 3b). corresponding amorphous NS conductivity is σ=7.12×10-2 Ω-1m-1.

At Vbias=2 V, the

Additionally, no hysteretic

behavior is evident.

Figure 3. (a) SEM image of a MoDTC NS device. (b) I-V response of a NS device in its crystalline and amorphous forms. (c) Temperature-dependent conductivity of the same device when made progressively more amorphous through prolonged electron beam exposure (IBeam = 7.1×10-4 A/cm2, 200 keV). The shaded region above 360 K shows evidence of Arrhenius behavior. In all cases, dashed lines are guides to the eye. Inset: Temperature-dependent conductivity of a crystalline NS device. To establish the origin of this dramatic conductivity enhancement, additional temperaturedependent I-V measurements were conducted. The inset in Figure 3c shows σ plotted against 1/T for a crystalline NS device. Clear Arrhenius behavior is observed, suggesting activated transport through trap states. Subsequent analyses of data taken at different Vbias values (Figure S12) show that this trap-limited conduction involves Poole-Frenkel emission since increasing Vbias from 1.0 V to 2.0 V lowers the apparent transport activation energy from 213 meV to 179 meV. A corresponding average intertrap distance (∆z) can be estimated from the slope (m) of the crystalline NS data in log (I)–V plots, where

∆ 29 .

In the expression, q is the


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electron charge, kB is the Boltzmann constant, T is the temperature and l is the inter-electrode spacing. We find ∆z ≈ 140 nm, which, in turn, suggests an associated crystalline NS (volume) trap density of Ntrap~1014 cm-3. Next, temperature-dependent measurements show that amorphous NS devices exhibit a linear log(σ) versus 1/T 1/3 dependency between 110 K and 360 K. Apparent Arrhenius behavior is seen above 360 K. Figure 3c shows representative data for Vbias=1 V where the crystalline NS device first highlighted in the inset is made progressively more amorphous. The linear log(σ) versus 1/T

1/3

behavior is characteristic of variable range hopping (VRH) and is supported by the excellent fit of the data to σ=A exp[-(B/T 1/n+1)] as well as with the observed ohmic response. In the expression, A is a normalization constant, B is the Mott temperature, associated with the density of trap states near the Fermi level, and n is the dimensionality of the transport.30,31 The 1/T 1/3 (i.e. n=2) dependency thus suggests two dimensional VRH in amorphous NSs. Additionally, by analyzing B, we find an associated 2D density of states near the Fermi level of N2D~1014 cm-2eV-1. An associated Mott hopping distance (Rh) is Rh=(a/3)(B/T 1/3)32 where a is a localization length that takes values on the order of 1 nm.33,34,35 Given B from earlier fits to the data (B≈94 K 1/3), we find Rh=4.5 nm. An associated (volume) trap density is Ntrap~1019 cm-3. It is therefore apparent that Ntrap increases by 5 orders of magnitude over the corresponding crystalline NS value. This suggests that the enhanced conductivity of amorphous MoDTC NSs stems from an amorphization-induced increase in the density of defect states available for VRH. We note that this behavior—where the amorphous state is significantly more conductive than the corresponding crystalline phase—is in dramatic contrast to the behavior generally observed in classical phase transition materials.36 To identify the defect states most likely responsible for the observed amorphous conductivity enhancement, electronic structures of both crystalline and amorphous MoDTC were computed using


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the local density approximation with a positive on-site interaction energy (LDA+U).37

For

crystalline MoDTC, projected density of states (p-DOS) plots (Figure S13a) show that the conduction band (CB) predominantly arises from Mo 4d-orbitals with an associated Mo oxidation state of 6+.

The corresponding valence band (VB) predominantly arises from S 3p-orbitals.

Crystalline, defect-free, MoDTC therefore possesses a computationally-estimated band gap of 2.8 eV. To account for the trap-mediated transport of crystalline NSs (Figure 3c), defects were introduced into the lattice by removing a sulfur atom from MoDTC’s carbamate tail (Figure S6b). Subsequent LDA+U calculations show that a sulfur 3p state migrates into the gap 0.69 eV below the Fermi level (Svac). A corresponding Mo-4d state with a 5+ oxidation number also appears 0.82 eV above the Fermi energy (Figure S13b). The corresponding band gap therefore decreases to 2.74 eV in good agreement with the experimentally-estimated gap energy of 2.72 eV (Figure S14). Near identical behavior is seen when sulfur vacancies are introduced into MoDTC’s Mo2S2O2 core (Figures S15 and S16). The good agreement between experimental and theoretical band gaps, sub gap transition energies, and molybdenum oxidation state therefore point to sulfur vacancies as most likely responsible for the activated transport in crystalline MoDTC. Comparisons of theoretical and experimental absorption spectra can be found in SI. At this point, to evaluate the effects of amorphization, crystalline structures with sulfur defects were stochastically quenched. Figure S13c shows that amorphization broadens both CB and VB states in p-DOS plots. The effective band gap decreases by 0.15 eV to 2.59 eV. The broadening also causes defect and band states to overlap. This blueshifts the Mo5+–Svac transition by 0.40 eV and, more relevantly, enhances its transition strength. Note that the predicted Mo5+–Svac blueshift is not observed experimentally. The simulation therefore qualitatively reproduces the enhancement of the


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sub gap feature in amorphous MoDTC absorption spectra (Figure S14). We thus posit that sulfur vacancies/corresponding Mo 4d gap states and electron beam-induced increases in their concentration are responsible for the enhanced conductivity of amorphized MoDTC NSs.

Summary In summary, large, free-standing, MoDTC NSs, which exhibit a crystalline-to-amorphous phase transition, have been synthesized. Both electron beam-induced ionization and fs optical excitation induce the phase transition, which is size-, morphology- and composition-preserving. What results are NSs which are the largest, regularly-shaped amorphous two dimensional nanostructures made to date.

Furthermore, the observed phase transition is accompanied by a dramatic conductivity

enhancement that exceeds 5 orders of magnitude with no gross changes to atomic coordination number or oxidation state. Subsequent temperature-dependent transport measurements suggest that this enhancement stems from the amorphization-induced formation of defect states, which results in efficient variable range hopping. Complementary LDA+U calculations suggest sulfur vacancies as the source of states responsible for the observed, defect-mediated, conductivity enhancement. This simultaneously leads to the appearance of an absorption feature to the red of the optical band gap, which itself remains largely unaltered. All of this, contrasts to classical phase change materials where the origin of a conductivity contrast stems from increases in the density of localized mid gap states upon amorphization as well as from local changes in atomic coordination number and a widening of the optical/transport gap.1 The observed structural phase transition, the accompanying changes in electrical response, and the fact that MoDTC belongs to a broader class of molybdenum carbamates thus suggests that MoDTC or a related metal carbamate may have eventual use as phase change materials for optical and


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electrical data storage. Beyond this, results of the current study add to growing knowledge about the intriguing and potentially useful properties of amorphous low-dimensional materials.

ASSOCIATED CONTENT Supporting Information. Mo(Et2CNS2)4 synthesis, MoDTC NS synthesis, mass spectra of Mo(Et2CNS2)4, additional MoDTC NS TEM images, MoDTC NS sizing histograms, MoDTC NS XRD analysis, schematic of MoDTC NS growth direction and crystal planes, MoDTC NS EXAFS analysis, SAED images showing MoDTC NS amorphization, optical images showing partial NS amorphization as well as corresponding Raman spectra, optical images of NSs before and after amorphization, XPS analyses of crystalline and amorphous NSs, thermal transport measurements, ∆T estimate under electron beam irradiation, details about transport measurements, discussion of trapassisted conduction in crystalline MoDTC NSs, analysis of crystalline NS I-V curves at different Vbias, LDA+U calculations.

This material is available free of charge via the Internet at

http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *MZ: [email protected] *MK: [email protected] Present Addresses †

Elgin Community College, 1700 Spartan Drive, Elgin, IL 60123, United States.

Funding Sources


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Army Research Office (W911NF-12-1-0578)

ACKNOWLEDGMENT This work was supported by the Army Research Office via award W911NF-12-1-0578. M.Z. thanks the Notre Dame Radiation Laboratory for partial financial support. A.R. acknowledges support from a NASA Space Technology Research Fellowship. The authors also thank ND Energy’s Materials Characterization Facility (MCF), the Notre Dame Mass Spectrometry and Proteomics Facility, the Notre Dame Integrated Imaging Facility (NDIIF), and the Notre Dame Radiation Laboratory for use of their facilities and equipment. We also thank the MRCAT10 Sector of the Advanced Photon Sources (APS), Argonne National Laboratory (ANL, IL) for EXAFS and XANES measurements. Finally, we thank Dr. W.C. Boggess, Prof. S. Rouvimov, and Prof. S. Ptasinska for helpful scientific discussions.

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(31) Kaiser, A. B.; Cristina, G. N.; Sundaram, R. S.; Burghard, M.; Kern, K. Nano Lett. 2009, 9 (5), 1787-1792. (32) Tsigankov, D. N.; Efros, A. L. Phys. Rev. Lett. 2002, 88 (2), 176602 (1-4). (33) Marianer, S.; Shklovskii, B. I. Phys. Rev. B 1992, 46 (20) 13100-13103. (34) Knotek, M. L. Solid State Commun. 1975, 17 (11), 1431-1433. (35) Ilie, A.; Harel, O.; Conway, N. M. J.; Yagi, T.; Robertson, J.; Milne, W. I. J. Appl. Phys. 2000, 87 (2), 789-794. (36) Siegrist, T.; Jost, P.; Volker, H.; Woda, M.; Merkelbach, P.; Schlockermann C.; Wuttig, M. Nature Mater. 2011, 10 (3), 202-208. (37) Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Humphreys, C. J.; Sutton, A. P. Phys. Rev. B 1998, 57, 1505-1509.


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Figure 1. (a) Low-magnification TEM image of MoDTC NSs. (b) AFM topography and linescan (inset) of an individual MoDTC NS. 84x42mm (300 x 300 DPI)


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Figure 2. (a) False color TEM images and SAED of MoDTC NSs before and after amorphization. (b) Ensemble absorption spectra of MoDTC NSs before (solid black line) and after (dashed red line) fs optical irradiation. Amorphization time constant from (c) electron beam energy and (d) current density measurements. 177x71mm (300 x 300 DPI)


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Figure 3. (a) SEM image of a MoDTC NS device. (b) I-V response of a NS device in its crystalline and amorphous forms. (c) Temperature-dependent conductivity of the same device when made progressively more amorphous through prolonged electron beam exposure (IBeam = 7.1x10-4 A/cm2, 200 keV). The shaded region above 360 K shows evidence of Arrhenius behavior. In all cases, dashed lines are guides to the eye. Inset: Temperature-dependent conductivity of a crystalline NS device. 187x52mm (300 x 300 DPI)


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TOC 84x35mm (300 x 300 DPI)


Molybdenum Carbamate Nanosheets as a New Class of Potential

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