High electrical conductivity and breakdown current density of individual monolayer Ti3C2Tx MXene flakes

Abstract

•Improved electrical conductivity of monolayer Ti3C2Tx MXene of up to 11,000 S cm−1•Improved mobility of monolayer Ti3C2Tx MXene of up to 6 cm2 V−1 s−1•High breakdown current density of monolayer Ti3C2Tx MXene of 1.2 × 108 A cm−2•Current annealing of Ti3C2Tx devices yields better electronic characteristics In recent years, the increasing demand for higher performance of integrated circuits has been met by scaling down various device components, including on-chip interconnects. However, as the use of conventional metals, such as copper, in miniaturized interconnects becomes increasingly challenging, there is a growing interest in alternative interconnect materials with high electrical conductivity and breakdown current density. Here, we demonstrate a very high breakdown current density in monolayer Ti3C2Tx, a material from the family of two-dimensional transition metal carbides known as MXenes, which exceeds such properties of copper and other conventional metals. The remarkable combination of high electrical conductivity and breakdown current density found in Ti3C2Tx extends the already impressive list of potential applications of MXenes to microelectronics and warrants investigation of other materials from the large MXene family, some of which may possess even better characteristics. As the scaling down of integrated circuits continues, there is a growing interest in electrically conductive materials with high current-carrying capacity for next-generation on-chip interconnects. Here, we report very high breakdown current density in Ti3C2Tx MXene, an emerging two-dimensional material. We performed electrical measurements of individual high-quality monolayer Ti3C2Tx flakes, which were prepared by an improved synthesis method, and found that they exhibit electrical conductivities of up to 11,000 S cm−1 and field-effect electron mobilities of up to 6 cm2 V−1 s−1, both representing the best values reported for Ti3C2Tx flakes so far. All flakes exhibited very similar breakdown current densities of about 1.2 × 108 A cm−2, which are comparable with the best two-dimensional materials, including graphene. The remarkable combination of high electrical conductivity and high current-carrying capacity makes Ti3C2Tx promising for nanometer-thin interconnects and warrants investigation of breakdown current densities of other materials from the large MXene family. As the scaling down of integrated circuits continues, there is a growing interest in electrically conductive materials with high current-carrying capacity for next-generation on-chip interconnects. Here, we report very high breakdown current density in Ti3C2Tx MXene, an emerging two-dimensional material. We performed electrical measurements of individual high-quality monolayer Ti3C2Tx flakes, which were prepared by an improved synthesis method, and found that they exhibit electrical conductivities of up to 11,000 S cm−1 and field-effect electron mobilities of up to 6 cm2 V−1 s−1, both representing the best values reported for Ti3C2Tx flakes so far. All flakes exhibited very similar breakdown current densities of about 1.2 × 108 A cm−2, which are comparable with the best two-dimensional materials, including graphene. The remarkable combination of high electrical conductivity and high current-carrying capacity makes Ti3C2Tx promising for nanometer-thin interconnects and warrants investigation of breakdown current densities of other materials from the large MXene family. Two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides, known as MXenes, are a large family of 2D materials with applications in energy storage, electronics, catalysis, and other fields.1Gogotsi Y. Anasori B. The rise of MXenes.ACS Nano. 2019; 13: 8491-8494Crossref PubMed Scopus (537) Google Scholar,2Anasori B. Gogotsi Y. 2D Metal Carbides and Nitrides (MXenes): Structure, Properties and Applications. Springer International Publishing, 2019Crossref Scopus (114) Google Scholar MXenes have a general formula of Mn+1XnTx, where M is a transition metal, such as Ti, Zr, Nb, V, etc.; X is carbon and/or nitrogen; n = 1, 2, 3, or 4; and Tx represents the surface functional groups.1Gogotsi Y. Anasori B. The rise of MXenes.ACS Nano. 2019; 13: 8491-8494Crossref PubMed Scopus (537) Google Scholar, 2Anasori B. Gogotsi Y. 2D Metal Carbides and Nitrides (MXenes): Structure, Properties and Applications. Springer International Publishing, 2019Crossref Scopus (114) Google Scholar, 3Deysher G. Shuck C.E. Hantanasirisakul K. Frey N.C. Foucher A.C. Maleski K. Sarycheva A. Shenoy V.B. Stach E.A. Anasori B. et al.Synthesis of Mo4VAlC4 MAX phase and two-dimensional Mo4VC4 MXene with five atomic layers of transition metals.ACS Nano. 2020; 14: 204-217Crossref PubMed Scopus (179) Google Scholar Remarkably, while more than 30 different MXenes have been experimentally demonstrated and many others have been predicted theoretically, over 50% of all research efforts so far have been focused on the first discovered MXene, Ti3C2Tx.4Naguib M. Kurtoglu M. Presser V. Lu J. Niu J. Heon M. Hultman L. Gogotsi Y. Barsoum M.W. Two-dimensional nanocrystals produced by exfoliation of Ti3AlC2.Adv. Mater. 2011; 23: 4248-4253Crossref PubMed Scopus (4767) Google Scholar The great interest in this material is related to the availability of well-established synthesis procedures,5Alhabeb M. Maleski K. Anasori B. Lelyukh P. Clark L. Sin S. Gogotsi Y. Guidelines for synthesis and processing of two-dimensional titanium carbide (Ti3C2Tx MXene).Chem. Mater. 2017; 29: 7633-7644Crossref Scopus (1637) Google Scholar its composition based on earth-abundant elements, as well as extraordinary physical and chemical properties. For example, electrical measurements of individual Ti3C2Tx flakes demonstrated their high electrical conductivity of about 4,600 S cm−1,6Lipatov A. Alhabeb M. Lukatskaya M.R. Boson A. Gogotsi Y. Sinitskii A. Effect of synthesis on quality, electronic properties and environmental stability of individual monolayer Ti3C2 MXene flakes.Adv. Electron. Mater. 2016; 2: 1600255Crossref Scopus (729) Google Scholar while nanoindentation experiments on monolayer Ti3C2Tx revealed its effective Young's modulus of 330 GPa, which exceeds the values reported for most other 2D materials.7Lipatov A. Lu H. Alhabeb M. Anasori B. Gruverman A. Gogotsi Y. Sinitskii A. Elastic properties of 2D Ti3C2Tx MXene monolayers and bilayers.Sci. Adv. 2018; 4: eaat0491Crossref PubMed Scopus (349) Google Scholar Interestingly, the combination of high electrical conductivity,6Lipatov A. Alhabeb M. Lukatskaya M.R. Boson A. Gogotsi Y. Sinitskii A. Effect of synthesis on quality, electronic properties and environmental stability of individual monolayer Ti3C2 MXene flakes.Adv. Electron. Mater. 2016; 2: 1600255Crossref Scopus (729) Google Scholar which shows only a weak temperature dependence,8Miranda A. Halim J. Barsoum M.W. Lorke A. Electronic properties of freestanding Ti3C2Tx MXene monolayers.Appl. Phys. Lett. 2016; 108: 033102Crossref Scopus (118) Google Scholar and stability suggests that Ti3C2Tx could possess a high breakdown current density, a technologically important property that has not been discussed with respect to MXenes yet. High current-carrying capacity may extend the already impressive list of potential applications of MXenes to new areas that include high-voltage technologies and on-chip interconnects. As the scaling down of integrated circuits continues, the use of conventional metals, such as copper, in miniaturized interconnects becomes increasingly challenging, because the resistance of metallic wires rises rapidly as their width decreases.9Steinhögl W. Schindler G. Steinlesberger G. Engelhardt M. Size-dependent resistivity of metallic wires in the mesoscopic range.Phys. Rev. B. 2002; 66: 075414Crossref Scopus (442) Google Scholar,10Kapur P. McVittie J.P. Saraswat K.C. Technology and reliability constrained future copper interconnects. I. Resistance modeling.IEEE Trans. Electron Devices. 2002; 49: 590-597Crossref Scopus (205) Google Scholar As a result, there is a growing interest in alternative interconnect materials, and many 2D materials with appreciable electrical conductivities have been tested for interconnect applications. Graphene, the first 2D material that received a wealth of attention from the materials community,11Geim A.K. Novoselov K.S. The rise of graphene.Nat. 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High current density and low thermal conductivity of atomically thin semimetallic WTe2.ACS Nano. 2016; 10: 7507-7514Crossref PubMed Scopus (81) Google Scholar ZrTe3 (∼108 A cm−2),19Geremew A. Bloodgood M.A. Aytan E. Woo B.W.K. Corber S.R. Liu G. Bozhilov K. Salguero T.T. Rumyantsev S. Rao M.P. et al.Current carrying capacity of quasi-1D ZrTe3 van der Waals nanoribbons.IEEE Electron Device Lett. 2018; 39: 735-738Crossref Scopus (44) Google Scholar and TaSe3 (∼108 A cm−2),20Empante T.A. Martinez A. Wurch M. Zhu Y. Geremew A.K. Yamaguchi K. Isarraraz M. Rumyantsev S. Reed E.J. Balandin A.A. et al.Low resistivity and high breakdown current density of 10 nm diameter van der Waals TaSe3 nanowires by chemical vapor deposition.Nano Lett. 2019; 19: 4355-4361Crossref PubMed Scopus (32) Google Scholar for some of which the reported values also exceeded such properties of copper and other conventional metals and were comparable with the characteristics of graphene. Considering the remarkable combination of physical properties of Ti3C2Tx and the demonstrated promise of other 2D materials for interconnect applications, in this study, we fabricated field-effect transistor (FET) devices based on monolayer Ti3C2Tx flakes and tested their current-carrying capacity to determine how they compare with 2D materials with the highest breakdown current densities reported so far. We utilized Ti3C2Tx prepared by the recently reported synthesis method,21Mathis T. Maleski K. Goad A. Sarycheva A. Anayee M. Foucher A.C. et al.Modified MAX phase synthesis for environmentally stable and highly conductive Ti3C2 MXene.ChemRxiv. 2020; https://doi.org/10.26434/chemrxiv.12805280.v1Crossref Scopus (0) Google Scholar which yields high-quality monolayer flakes with a Ti:C atomic ratio very close to the ideal value of 1.5 and improved chemical stability. The devices exhibited electrical conductivities of up to 11,000 S cm−1 and field-effect electron mobilities of up to 6 cm2 V−1 s−1, both representing an improvement by a factor of two compared with the values reported in our previous study of similar Ti3C2Tx FETs.6Lipatov A. Alhabeb M. Lukatskaya M.R. Boson A. Gogotsi Y. Sinitskii A. Effect of synthesis on quality, electronic properties and environmental stability of individual monolayer Ti3C2 MXene flakes.Adv. Electron. Mater. 2016; 2: 1600255Crossref Scopus (729) Google Scholar As we increased the drain-source voltages applied to the Ti3C2Tx devices, we first observed the improvement in electrical conductivity, which we attribute to the effect of joule heating leading to removal of adsorbed species.22Moser J. Barreiro A. Bachtold A. Current-induced cleaning of graphene.Appl. Phys. Lett. 2007; 91: 163513Crossref Scopus (529) Google Scholar This result suggests that current annealing is a useful procedure for improving electrical characteristics of MXene devices. At higher voltages, we observed electrical breakdown of Ti3C2Tx devices, which was confirmed by microscopic techniques. All tested devices exhibited very similar breakdown current densities of about 1.2 × 108 A cm−2, which are comparable with the best 2D materials, including graphene. The remarkable combination of high electrical conductivity and high current-carrying capacity makes Ti3C2Tx promising for interconnect applications and warrants investigation of breakdown current densities of other materials from the large MXene family, some of which may possess even better characteristics. MXenes are synthesized from layered carbides and nitrides with a general formula of Mn+1AXn, known as MAX phases,1Gogotsi Y. Anasori B. The rise of MXenes.ACS Nano. 2019; 13: 8491-8494Crossref PubMed Scopus (537) Google Scholar, 2Anasori B. Gogotsi Y. 2D Metal Carbides and Nitrides (MXenes): Structure, Properties and Applications. Springer International Publishing, 2019Crossref Scopus (114) Google Scholar and the synthesis protocol plays a crucial role in the functional properties and the environmental stability of the produced material.6Lipatov A. Alhabeb M. Lukatskaya M.R. Boson A. Gogotsi Y. Sinitskii A. Effect of synthesis on quality, electronic properties and environmental stability of individual monolayer Ti3C2 MXene flakes.Adv. Electron. Mater. 2016; 2: 1600255Crossref Scopus (729) Google Scholar Reagents used for the MAX phase etching may change the terminal groups (=O, –OH, –F, etc.) in the resulting MXene, altering its electronic properties. Also, harsh reagents can promote the formation of multiple defects in the MXene flakes, which adversely affect their stability in water. Finally, the quality of MXene depends on the crystallinity and grain size of the precursor MAX phase, which are strongly affected by crystal growth conditions. In this work, we synthesized MXene using an improved method presented by Mathis et al.,21Mathis T. Maleski K. Goad A. Sarycheva A. Anayee M. Foucher A.C. et al.Modified MAX phase synthesis for environmentally stable and highly conductive Ti3C2 MXene.ChemRxiv. 2020; https://doi.org/10.26434/chemrxiv.12805280.v1Crossref Scopus (0) Google Scholar which produces flakes with significantly better environmental stability compared to the previously reported procedures.[5Alhabeb M. Maleski K. Anasori B. Lelyukh P. Clark L. Sin S. Gogotsi Y. Guidelines for synthesis and processing of two-dimensional titanium carbide (Ti3C2Tx MXene).Chem. Mater. 2017; 29: 7633-7644Crossref Scopus (1637) Google Scholar] This method relies on the use of a precursor Ti3AlC2 MAX phase with improved grain size, stoichiometry, and crystallinity, which was produced using an excess of Al and Ti in the mixture with TiC. After a solid-state synthesis at 1,380°C, the Ti3AlC2 particles were washed with HCl and then subjected to selective etching of Al using HF/HCl solution, followed by a delamination to produce large flakes, up to 20 μm in size. It was shown that these Ti3C2Tx MXene flakes have a Ti:C atomic ratio very close to the ideal value of 1.5,21Mathis T. Maleski K. Goad A. Sarycheva A. Anayee M. Foucher A.C. et al.Modified MAX phase synthesis for environmentally stable and highly conductive Ti3C2 MXene.ChemRxiv. 2020; https://doi.org/10.26434/chemrxiv.12805280.v1Crossref Scopus (0) Google Scholar and a lower concentration of structural defects when compared with the flakes made by other methods.5Alhabeb M. Maleski K. Anasori B. Lelyukh P. Clark L. Sin S. Gogotsi Y. Guidelines for synthesis and processing of two-dimensional titanium carbide (Ti3C2Tx MXene).Chem. Mater. 2017; 29: 7633-7644Crossref Scopus (1637) Google Scholar As a result, the Ti3C2Tx flakes prepared by this new method had a very long shelf life of several months if stored as aqueous suspensions in ambient conditions, and their oxidation in air started at temperatures 100°C–150°C higher than for similar MXenes produced from a conventional Ti3AlC2.21Mathis T. Maleski K. Goad A. Sarycheva A. Anayee M. Foucher A.C. et al.Modified MAX phase synthesis for environmentally stable and highly conductive Ti3C2 MXene.ChemRxiv. 2020; https://doi.org/10.26434/chemrxiv.12805280.v1Crossref Scopus (0) Google Scholar The structure of a Ti3C2Tx MXene flake is shown in Figure 1A, demonstrating three layers of Ti in the cubic close packed (ccp) arrangement with carbon atoms occupying the octahedral voids. The Ti3C2Tx structure in Figure 1A is oriented in a way that highlights its relationship with the NaCl-type structure of TiC.23Lipatov A. Alhabeb M. Lu H. Zhao S. Loes M.J. Vorobeva N.S. Dall'Agnese Y. Gao Y. Gruverman A. Gogotsi Y. et al.Electrical and elastic properties of individual single-layer Nb4C3Tx MXene flakes.Adv. Electron. Mater. 2020; 6: 1901382Crossref Scopus (47) Google Scholar While the surface of the Ti3C2Tx flake in Figure 1A is schematically shown to be terminated with hydroxyl groups, other terminal moieties, such as =O and –F, were also shown to be present in Ti3C2Tx based on the results of X-ray photoelectron spectroscopy (XPS).24Halim J. Cook K.M. Naguib M. Eklund P. Gogotsi Y. Rosen J. Barsoum M.W. X-ray photoelectron spectroscopy of select multi-layered transition metal carbides (MXenes).Appl. Surf. Sci. 2016; 362: 406-417Crossref Scopus (800) Google Scholar The synthesized Ti3C2Tx MXene was stored as a colloidal solution in water, which prevented the flakes from aggregating. It was shown that Ti3C2Tx flakes synthesized by the improved method can be stored as an aqueous suspension for at least 10 months with only minor degradation of electronic properties.21Mathis T. Maleski K. Goad A. Sarycheva A. Anayee M. Foucher A.C. et al.Modified MAX phase synthesis for environmentally stable and highly conductive Ti3C2 MXene.ChemRxiv. 2020; https://doi.org/10.26434/chemrxiv.12805280.v1Crossref Scopus (0) Google Scholar A photograph of an aqueous colloidal solution of Ti3C2Tx flakes is presented in Figure 1B. The solution has the green color that is characteristic of Ti3C2Tx MXene.5Alhabeb M. Maleski K. Anasori B. Lelyukh P. Clark L. Sin S. Gogotsi Y. Guidelines for synthesis and processing of two-dimensional titanium carbide (Ti3C2Tx MXene).Chem. Mater. 2017; 29: 7633-7644Crossref Scopus (1637) Google Scholar,25Hantanasirisakul K. Alhabeb M. Lipatov A. Maleski K. Anasori B. Salles P. Ieosakulrat C. Pakawatpanurut P. Sinitskii A. May S.J. et al.Effects of synthesis and processing on optoelectronic properties of titanium carbonitride MXene.Chem. Mater. 2019; 31: 2941-2951Crossref Scopus (83) Google Scholar This color is consistent with the results of optical spectroscopy of an aqueous colloidal solution of Ti3C2Tx (Figure 1C), where the minimum absorption in the visible range of spectrum is observed around 520 nm (green). The ultraviolet-visible-near-infrared (UV-vis-NIR) absorption spectrum in Figure 1C is consistent with prior reports for Ti3C2Tx MXene.25Hantanasirisakul K. Alhabeb M. Lipatov A. Maleski K. Anasori B. Salles P. Ieosakulrat C. Pakawatpanurut P. Sinitskii A. May S.J. et al.Effects of synthesis and processing on optoelectronic properties of titanium carbonitride MXene.Chem. Mater. 2019; 31: 2941-2951Crossref Scopus (83) Google Scholar When a concentrated MXene suspension is dried on a substrate, the flakes are primarily deposited parallel to the surface, forming a layered film that is schematically shown in the inset in Figure 1D. The structure of this layered film can be probed by X-ray diffraction (XRD) analysis, as presented in Figure 1D. The XRD spectrum of a layered Ti3C2Tx MXene film in the range of 5° ≤ 2θ ≤ 50° shows six peaks, which can be indexed as a series of 00l (l = 1, 2, … 6) reflections. Since no other peaks are found in the XRD spectrum, we conclude that the sample does not contain appreciable quantities of crystalline impurities, including the original Ti3AlC2 MAX phase. The indexed reflections correspond to the spacing of 1.25 nm, which is larger than the nominal Ti3C2Tx monolayer thickness of 0.98 nm.7Lipatov A. Lu H. Alhabeb M. Anasori B. Gruverman A. Gogotsi Y. Sinitskii A. Elastic properties of 2D Ti3C2Tx MXene monolayers and bilayers.Sci. Adv. 2018; 4: eaat0491Crossref PubMed Scopus (349) Google Scholar This difference implies the presence of the water molecules trapped between the layers, which is in line with the previously reported data on Ti3C2Tx synthesized using the minimally intensive layer delamination (MILD) method.6Lipatov A. Alhabeb M. Lukatskaya M.R. Boson A. Gogotsi Y. Sinitskii A. Effect of synthesis on quality, electronic properties and environmental stability of individual monolayer Ti3C2 MXene flakes.Adv. Electron. Mater. 2016; 2: 1600255Crossref Scopus (729) Google Scholar The results of XPS characterization of drop-casted MXene films were consistent with the previously published data for Ti3C2Tx.6Lipatov A. Alhabeb M. Lukatskaya M.R. Boson A. Gogotsi Y. Sinitskii A. Effect of synthesis on quality, electronic properties and environmental stability of individual monolayer Ti3C2 MXene flakes.Adv. Electron. Mater. 2016; 2: 1600255Crossref Scopus (729) Google Scholar,24Halim J. Cook K.M. Naguib M. Eklund P. Gogotsi Y. Rosen J. Barsoum M.W. X-ray photoelectron spectroscopy of select multi-layered transition metal carbides (MXenes).Appl. Surf. Sci. 2016; 362: 406-417Crossref Scopus (800) Google Scholar,26Peng C. Yang X. Li Y. Yu H. Wang H. Peng F. Hybrids of two-dimensional Ti3C2 and TiO2 exposing {001} facets toward enhanced photocatalytic activity.ACS Appl. Mater. Interfaces. 2016; 8: 6051-6060Crossref PubMed Scopus (418) Google Scholar A high-resolution XPS Ti2p spectrum (Figure 1E) was collected using the pass energy of 20 eV and the step of 0.1 eV. It can be fit with three pairs of components: (1) Ti-C belonging to pure MXene phase (455.4 eV for Ti2p3/2 and 460.9 eV for Ti2p1/2);6Lipatov A. Alhabeb M. Lukatskaya M.R. Boson A. Gogotsi Y. Sinitskii A. Effect of synthesis on quality, electronic properties and environmental stability of individual monolayer Ti3C2 MXene flakes.Adv. Electron. Mater. 2016; 2: 1600255Crossref Scopus (729) Google Scholar,24Halim J. Cook K.M. Naguib M. Eklund P. Gogotsi Y. Rosen J. Barsoum M.W. X-ray photoelectron spectroscopy of select multi-layered transition metal carbides (MXenes).Appl. Surf. Sci. 2016; 362: 406-417Crossref Scopus (800) Google Scholar,26Peng C. Yang X. Li Y. Yu H. Wang H. Peng F. Hybrids of two-dimensional Ti3C2 and TiO2 exposing {001} facets toward enhanced photocatalytic activity.ACS Appl. Mater. Interfaces. 2016; 8: 6051-6060Crossref PubMed Scopus (418) Google Scholar (2) Ti–X from sub-stoichiometric TiCx (x < 1) or titanium oxycarbides (456.0 eV for Ti2p3/2 and 461.9 eV for Ti2p1/2);26Peng C. Yang X. Li Y. Yu H. Wang H. Peng F. Hybrids of two-dimensional Ti3C2 and TiO2 exposing {001} facets toward enhanced photocatalytic activity.ACS Appl. Mater. Interfaces. 2016; 8: 6051-6060Crossref PubMed Scopus (418) Google Scholar (3) Ti bound to the oxygen surface functional group, which could be represented as TixOy (457.2 eV for Ti2p3/2 and 462.8 eV for Ti2p1/2).26Peng C. Yang X. Li Y. Yu H. Wang H. Peng F. Hybrids of two-dimensional Ti3C2 and TiO2 exposing {001} facets toward enhanced photocatalytic activity.ACS Appl. Mater. Interfaces. 2016; 8: 6051-6060Crossref PubMed Scopus (418) Google Scholar However, the main result of the XPS characterization is the absence of signals from TiO2 (458.8 eV for Ti2p3/2),6Lipatov A. Alhabeb M. Lukatskaya M.R. Boson A. Gogotsi Y. Sinitskii A. Effect of synthesis on quality, electronic properties and environmental stability of individual monolayer Ti3C2 MXene flakes.Adv. Electron. Mater. 2016; 2: 1600255Crossref Scopus (729) Google Scholar,24Halim J. Cook K.M. Naguib M. Eklund P. Gogotsi Y. Rosen J. Barsoum M.W. X-ray photoelectron spectroscopy of select multi-layered transition metal carbides (MXenes).Appl. Surf. Sci. 2016; 362: 406-417Crossref Scopus (800) Google Scholar,26Peng C. Yang X. Li Y. Yu H. Wang H. Peng F. Hybrids of two-dimensional Ti3C2 and TiO2 exposing {001} facets toward enhanced photocatalytic activity.ACS Appl. Mater. Interfaces. 2016; 8: 6051-6060Crossref PubMed Scopus (418) Google Scholar which further confirms the high quality of the studied Ti3C2Tx MXene sample. When a droplet of a diluted Ti3C2Tx solution in water was dried on a Si/SiO2 substrate, the size of randomly distributed MXene flakes could be examined by optical microscopy (Figure 1F). Uniformly colored flakes of up to 20 μm in size could be located and later used for device fabrication. Transmission electron microscopy (TEM) imaging allows a more detailed structural characterization of Ti3C2Tx. Shown in Figure 1G is an individual flake of about 5 μm in size that has a very uniform surface without visible pinholes or foreign particles, which suggests high quality of the MXene material. Partially oxidized Ti3C2Tx flakes are known to exhibit pinholes and elongated TiO2 particles, which tend to predominantly congregate at the edges; see examples of TEM images of degraded Ti3C2Tx flakes in Figure S1. High-resolution TEM (Figure 1H) shows the expected hexagonal arrangement of Ti atoms with the average interatomic distance of about 0.302 nm. Twelve two-terminal devices with Ti3C2Tx MXene channels were fabricated using the standard electron beam lithography followed by the deposition of 3 nm of Cr and 15 nm of Au. The device scheme is presented in Figure 2A, showing a monolayer Ti3C2Tx flake bridging source (S) and drain (D) electrodes on top of a Si/SiO2 substrate. The heavily p-doped Si substrate served as a bottom gate (G) electrode in the electrical measurements. A scanning electron microscopy (SEM) image of a representative Ti3C2Tx device is shown in Figure 2B; the Cr/Au electrodes are colored in yellow for clarity. The typical channel length in the fabricated devices was about 5 μm. The lateral dimensions of all studied devices are presented in Table S1 for reference. Electrical characterization of the devices was performed in vacuum at the residual pressure of 2 × 10−6 torr after 2 days of evacuation to minimize the effect of surface adsorbates.27Sinitskii A. Dimiev A. Kosynkin D.V. Tour J.M. Graphene nanoribbon devices produced by oxidative unzipping of carbon nanotubes.ACS Nano. 2010; 4: 5405-5413Crossref PubMed Scopus (103) Google Scholar The IDS-VDS dependencies at three different gate voltages, −40, 0, and 40 V, are shown in Figure 2C. The IDS-VDS dependencies are linear, which is indicative of ohmic contacts between the channel material and the gold electrodes, and nearly coincide, demonstrating a weak modulation of the electrical conductivity of Ti3C2Tx by the gate voltage. The sheet resistances for the devices were calculated from the IDS-VDS curves measured at VG = 0 and the flake dimensions, see Table S1. The average sheet resistance is 1,160 ± 220 Ω □−1. MXene resistivity can be estimated by dividing the sheet resistance by MXene thickness, which, according to TEM imaging and theoretical calculations, is about 0.98 nm.28Wang X. Shen X. Gao Y. Wang Z. Yu R. Chen L. Atomic-scale recognition of surface structure and intercalation mechanism of Ti3C2X.J. Am. Chem. Soc. 2015; 137: 2715-2721Crossref PubMed Scopus (339) Google Scholar,29Halim J. Lukatskaya M.R. Cook K.M. Lu J. Smith C.R. Näslund L.-Å. May S.J. Hultman L. Gogotsi Y. Eklund P. et al.Transparent conductive two-dimensional titanium carbide epitaxial thin films.Chem. Mat. 2014; 26: 2374-2381Crossref PubMed Scopus (750) Google Scholar The average resistivity for the nine measured monolayer MXene devices is 1.14 ± 0.21 μΩ m, with the minimum of about 0.9 μΩ m. These values correspond to the average conductivity of 9,050 ± 1,620 S cm−1 and the maximum conductivity of 11,000 S cm−1, respectively, representing a significant improvement over our previous result of 4,600 S cm−1, which was reported for Ti3C2Tx flakes synthesized by the MILD method.6Lipatov A. Alhabeb M. Lukatskaya M.R. Boson A. Gogotsi Y. Sinitskii A. Effect of synthesis on quality, electronic properties and environmental stability of individual monolayer Ti3C2 MXene flakes.Adv. Electron. Mater. 2016; 2: 1600255Crossref Scopus (729) Google Scholar Better electronic characteristics could be attributed to the higher quality of Ti3C2Tx flakes prepared by the improved synthesis method.21Mathis T. Maleski K. Goad A. Sarycheva A. Anayee M. Foucher A.C. et al.Modified MAX phase synthesis for environmentally stable and highly conductive Ti3C2 MXene.ChemRxiv. 2020; https://doi.org/10.26434/chemrxiv.12805280.v1Crossref Scopus (0) Google Scholar Figure 2D shows transfer characteristics measured in the range of −40 V to +40 V applied to the gate electrode. The IDS-VG dependence is linear and similar to our previous report.6Lipatov A. Alhabeb M. Lukatskaya M.R. Boson A. Gogotsi Y.