PERSPECTIVE The New Skinny in Two-Dimensional Nanomaterials Kristie J. Koski† and Yi Cui†,‡,*

†Department of Materials Science & Engineering, Stanford University, Stanford, California 94305, United States, and ‡Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States

ABSTRACT While the advent of has focused attention on the extraordinary properties of two-dimensional (2D) materials, graphene's lack of an intrinsic band gap and limited amenability to chemical modification has sparked increasing interest in its close relatives and in other 2D layered nanomaterials. In this issue of ACS Nano, Bianco et al. report on the production and characterization of one of these related materials: germanane, a one-atom-thick sheet of hydrogenated puckered atoms structurally similar to graphane. It is a 2D nanomaterial generated via mechanical exfoliation from GeH. Germanane has been predicted to have technologically relevant properties such as a direct band gap and high electron mobility. Monolayer 2D materials like germanane, in general, have attracted enormous interest for their potential technological applications. We offer a perspective on the field of 2D layered nanomaterials and the exciting growth areas and discuss where the new development of germanane fits in, now and in the foreseeable future.

gnited by immense technological pro- 2D electron gas physics, , mise, the field of two-dimensional (2D) spontaneous magnetization, and anisotropic layered nanomaterials has grown ex- transport properties. Layered 2D materials I 1,2 tensively over the past decade. Two- find current applications in a wide array of dimensional nanomaterials are typically capacities such as batteries, electrochromics, generated from bulk layered crystalline cosmetics, catalysts, and solid lubricants.3 solids (Figure 1) such as graphite or di- With successive thinning of the layers to chalcogenides. These solids consist of monolayer dimensions, the inherent prop- successive layers of covalently bonded erties of these bulk materials are altered. atomic layer planes ranging from one to As research in this field has grown, many multiple atoms thick, separated successively monolayer materials with unique physical, by van der Waals gaps. Single monolayers are electronic, and structural properties have generated via a variety of methods, primarily emerged. Exciting examples can be found mechanical exfoliation, liquid exfoliation, or in the numerous investigations of layered lithium-intercalation/deintercalation of these transition metal chalcogenides (MoS2, layered materials. WS2, TiSe2,Bi2Se3) approaching monolayer thicknesses.2 These materials, in particular, demonstrate high mobilities and preserve a In this issue of ACS Nano, band gap approaching few-layer systems. et al An exceptional example is MoS2, with a Bianco . report the 2 5 mobility of 200 cm /(V s). MoS2 also under- generation of stable, single- goes a phase change from an indirect to layered germanane. direct band gap semiconductor with accom- panying photoluminescence, drawing much * Address correspondence to interest as a possible 2D transistor material. [email protected].

Two-dimensional materials demonstrate Other metal chalcogenides, particularly Bi2Se3 2 a vast array of unique physical properties. (Figure 1), Sb2Te3,andBi2Te3,demonstrate Published online These include properties possessed by their both thermoelectric and topological insulator 10.1021/nn4022422 bulk, layered counterparts3 such as charge properties, which have attracted significant 4 4 density waves, topological insulator behavior, interest for future applications. C XXXX American Chemical Society

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a veritable playground for theoreti- cal physicists. But graphene does not have an intrinsic bandgap, and attempts to engineer a bandgap often reduce its mobility drastically. There has been much drive to de- velop graphene variants from similar -based materials and other 2D layered materials such as layered ni- trides, , and transition metal dichalcogenides. This has led to en- ormous developments of 2D mono- layer materials. Hydrogenation of graphene brought about graphane (Figure 2).7 Graphane is a fully saturated 2D hydrocarbon Figure 1. Examples of two-dimensional (2D) layered materials. An almost limitless of CH with sp3 array of layered materials exists from which 2D nanomaterials can be generated. The general structure of a 2D layered nanomaterial consists of a series of hybridized bonds that opens up a 8 covalently bonded atomic layers bound together by van der Waals forces. gap of 5.4 eV. Graphane retains the flexibility, the 2D flatness, and much of the strength of graphene, but it TABLE 1. Layered Crystal Structures That Have Been or May Potentially Be Mechanically Exfoliated is an insulator. Graphane lacks the Dirac cone of graphene. Both gra- Group IV Dichalcogenides Trichalcogenides Oxides Halides Potential 2D phene and graphane, however, lack Graphene C VSe , NbSe ,BiSe3, Bi Te , MoO ,VO , FeCl , Zintl Hosts 2 2 2 2 3 3 2 5 3 a direct band gap thus making these Graphane CH TiS , ZrS , HfS ,SbTe3, Bi S ,WO, ... FeBr , CaSi , CaGe , 2 2 2 2 2 3 3 3 2 2 materials not useful for most opto- CF ReS2, PtS2,In2Se3, As2S3, CrCl3, Ca(Si1‑xGex)2 electronic applications (Table 2). Silicene Si TiSe2, ZrSe2,As2Se3, NbSe3, Nitrides CrBr3, Ba3Sn4As6 fi Germanane GeH HfSe2, ReSe2, TiS3, ZrS3, ZrSe3, BN MoCl3, CaMg2N2 Speci cprogressingrapheneand PtSe2, SnSe2, ZrTe3, HfS3, MoBr3, Caln2 graphane has driven even greater TiTe2, ZrTe2, HfSe3, HfTe3, Oxychlorides TiCl2, CaNi2P2 interest in the semiconducting sili- MXenes VTe2,NbTe2, NbS3, TaS3, BiOCl, FeOCl, TiBr3, CaAuGa, ... con and germanium counterparts, Ti3C2,Ti2C, Ta4C3, TaTe2, MoTe2, TaSe3, ... HoOCl, ErOCl, InBr3, silicene and germanene. Both of Ti (Co. No.5)2, ... WTe , CoTe , ErOCl, TmOCl, Pbl , 3 5 2 2 2 these materials are predicted to RhTe2, IrTe2, Mono-Chalcogenides YbOCl, LnOCl, ... A1C133, have mixed sp2sp3 hybridization, NiTe2, PdTe2, GeSe, GeTe, Layered Silicate InBr33, PtTe2, SiTe2, GaSe, GaS Minerals CrBr33, which results in puckering (Table 2) NbS2, TaS2, Egyptian Blue, ... FeCl2, of the silicon and germanium atoms MoS2,WS2, Thiophosphates MgCl2, but retains the semiconducting char- TaSe2, MoSe2, FePS3, MnPS3, CoCl2, acteristics of the bulk material. Of WSe2, MoTe2, NiPS3, ... VC12, these, silicene has been demon- SnSe , SnS , ... VBr ,VI 2 2 2 2 strated experimentally via vapor CdCl , 2 growth on a silver substrate.9 It Cdl2, ... has interesting physical and elec- Other unique chemical ap- in 2004, a single atomic layer of tronic properties. Silicene shows proaches to develop new 2D ma- graphite with sp2 hybridized car- high mobility, a characteristic Dirac terials have also arisen. Treating bon.1 Graphene attracted tremen- cone, and a band gap that opens ternary carbides, nitrides, and car- dous interest due to its unique with, and is proportional to, an ap- bonitrides with fluoride properties such as thinness, high plied electric field.10 Silicene, how- (HF) gives rise to layered metal car- electrical and thermal conductivity, ever, requires a supporting layer bide and nitride systems. Materials, high mechanical strength, and high such as silver, zirconium diboride, dubbed MXenes, such as Ti3C2,Ti2C, mobility, yielding strong potential or iridium, all of which conduct elec-

Ta4C3, and Ti3(C0.5N0.5)2, have been for future electronics. Graphene is tricity, thereby eliminating the overall generated.6 An extensive list of bulk particularly interesting because the usefulness of silicene's properties.9 layered compounds that have been electronic band structure has a Dirac Consequently, these germanene or can be mechanically exfoliated is cone, the consequence of linear and silicene graphene analogues found in Table 1. dispersion in its electronic band have driven focus toward hydroge- The 2D nanomaterial field was diagram, which makes graphene a nated silicenes and germanenes. In fueled by the discovery of graphene high-mobility conductor and produces this issue of ACS Nano, Bianco et al.

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hydride:12

(CaGe2)n þ 2nHCl f (GeH)2n þ nCaCl2

The host CaGe2 matrix under- goes a structural change to a crys- talline covalently bonded solid of GeH, a layered hydrogenated ger- manium hydride. The bonding type appears to have mixed sp2 and sp3 Figure 2. The growing family of Group IV monolayers. Generating group IV hybridization.8,11 Layered GeH can layered compounds has garnered significant interest for potential technological be made in fairly substantial quan- value in the semiconducting industry. The latest of these new materials is germanane, developed recently by Biano et al. tities. Bianco et al. proceeded to use mechanical exfoliation to generate single layers of these atomically TABLE 2. Electronic Properties of Group IV (C, Si, Ge) Two-Dimensional Nanomaterials thin hydrogenated germaniums, dubbed germanane. These materi- Band Gap [eV] atomic puckering mobility [cm2/(V s)] classification als have remarkable resistance to Graphene 0 eV 0.00 Å ∼200,000 Semimetal19 oxidative degradation over a sub- Graphane 3.65.4 eV 0.46 Å - Insulator8 stantial period of five months. It was Silicene 0.0 eV 0.440.72 Å ∼105 Semimetal8 found that in layers of germanane, Germanane 1.532.4 eV 0.630.73 Å 18,195 Direct-Gap Semiconductor11 only the top layer would be oxi- dized, thus potentially protecting report the generation of stable, sin- dimensionality offer much promise the unique features of subsequent gle-layered germanane, which is a in a limitless array of technological stacked germanane layers. hydrogenated single layer of ger- applications ranging from thermo- Germanane is silver-black. Un- manium, a 2D graphane analogue electrics, transparent electrodes, like silicene, it does not require a material with a direct band gap.11 batteries, ultra-thin solar cells, and substrate to be stable. GeH has a Germanane is a graphane analogue high mobility electronic transition van der Waals gap of 5.9 Å;much that has been predicted to show devices, to new opto-electronic de- larger than similar layered struc- better theoretical use in electronics vices. The emerging 2D layered tures such as MoO3 and large en- applications because hydrogena- semiconducting graphene and gra- ough to serve as a host for the pos- tion destroys the Dirac cone and phane material analogues, such as sible intercalation of many different opens up a finite band gap. Bulk- germane, silicene, germanane, and species.3,16 The atomic structure hydrogen-saturated silicon and ger- silicane offer much exciting promise of germanane is puckered rather manium sheets have been synt- for future electronic materials. than flat like graphane or graphene hesized as layered polysilanes or Germanane. Bianco et al. report a (Figure 2). Calculations do not show polygermynes.8,12,13 Investigations unique method for generation of the existence of a Dirac cone but on those bulk layered materials have stable, single-layered germanane, germanane still has surprising high shown a direct band gap of 1.7 eV for the newest of the novel 2D nano- mobility;much higher than that polygermyneaswellasstrongopti- materials (Figure 2).11 Using topo- of its bulk material. Calculations cal photoluminescence at 1.35 eV.14 tactic deintercalation, the authors by Bianco et al. show a mobility of 2 All of these 2D materials, and par- converted β-CaGe2, a Zintl phase 18 200 cm /(V s), which is almost a ticularly Ge- or Si-based 2D materi- material, into a layered GeH by pla- 5-fold increase over that of bulk als with their nonzero band gaps, cing a large synthesized crystal into Ge (3900 cm2/(V s)). By comparison, offer significant promise in technolog- aqueous HCl at 40 °C. Zintl phase the mobility of suspended graphene ical areas such as thinner transistors, materials consist of an alkali or alka- is ∼200 000 cm2/(V s), while on a solar cells, photodetector materials, line metal (group 1,2) and a metal or substrate the value drops to 15 000 and thermoelectrics. These materials (group 13,14,15,16). In the cm2/(V s).14 It will be exciting to see are unique in exhibiting new com- CaGe2 Zintl phase materials, the future measurements of the elec- binations of fundamental physical crystal structure resembles that of tronic conductivity of germanane behavior such as photolumines- an intercalation layered compound. to confirm this high mobility. cence, a Dirac cone, and exceptional β-CaGe2 has alternating planes of Theoretical calculations and ab- transport properties. Approaching covalently bonded germanium atom sorption measurements show that 2D, the fundamental material physi- layers separated by ionically bonded germanane has a theoretical direct cal properties appear distinct from interstitial calcium. band gap of 1.53 eV, potentially those of the bulk materials. New Topotactic deintercalation ex- making it a viable material for properties tied with thin material tracts the calcium ion to form a solar cells. This value was roughly

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Figure 3. Developing chemistries of two-dimensional (2D) layered materials. Beyond various crystal growth methods, chemistry techniques specific to generating or altering 2D layered nanomaterials have arisen. These include mechanical exfoliation, new intercalation techniques, electrochemical exfoliation, heterostructure restructuring, and, now, topochemical deintercalation followed by mechanical exfoliation. confirmed with absorption measure- ion means the chemical surface can With germanane's structural an- ments, although it is understood that be further modified to adjust the isotropy, it will be interesting to see future angle-resolved photoelectron band gap, temperature dependent the layer dependence on the vibra- spectroscopy measurements will be stability, or other properties of the tional, optical, and electronic prop- needed to characterize the band material. For example, hydrogen erties. The anisotropic properties of structure fully. Band calculations of groups could be replaced by OH germanane could be very appeal- germanane do not show a Dirac cone groups to form single-layered germo- ing, potentially because of electron- similartothatofgrapheneorgra- xanene.12,15 It may be possible to use hole localization in one dimension. phane. Despite the optical bandgap, various ligands to influence the lumi- Bianco et al. demonstrate a 2D semi- no photoluminescence was observed, nescence properties and band gap conducting material with a strong possibly due to the presence of trace position. A similar example exists direct band gap, offering promise amounts of impurity GeCl. This is in bulk-layered polysiloxene, which for a world of electronics requiring quite surprising since polygermanes shows photoluminescence shifts thinner devices. In terms of specific (GeH) have previously shown strong with varying ligands.12 Functionali- applications, this material could be optical photoluminescence.15 Further zation may also be able to make this used in solar cells, transistor de- investigations may ultimately reveal material more stable with temperature. vices, or even like graphene, as a strong photoluminescence in this cooling layer. material. From a synthetic standpoint, The greatest drawback to this Despite drawbacks, the the topotactic deintercalation tech- new material, germanane, is its be- nique used to make germanane, havior with increased temperature. future of germanane coupled with mechanical or chemi- Whereas in graphane annealing itself may be limitless. cal exfoliation, may offer an exciting allows the hydrogen to disperse, thus new pathway toward many novel reverting graphane to graphene,7 2D materials. Topotactic deinterca- in germanane this is not the case. Because germanane derives from lation of only a minor number of Instead, it becomes amorphous a layered host structure, established Zintl phase compounds has been with dehydrogenation above 75 °C. techniques on layered materials could performed. This technique has been Thus, there remains some question of be utilized for further adjustment of used to make polygermananes, how well germanane could work in structural and behavioral properties. polysilanes, layered siloxenes, and electronic materials. Many transistors (The growing list of novel chemistries layered spinel materials. Many of can operate up to hundreds of degrees on 2D layered nanomaterials can be these layered products were sensi- Celsius, temperatures that would lead seen in Figure 3.) Germanane can tive to oxidation. For example, the to destruction of germanane-based be restacked with other materials. silicon-based Zintl phase material, electronics. Germanane-based elec- As a layered material, new and old CaSi2, quickly leads to siloxene, an tronics may require cooling or may intercalation techniques3 may offer oxidated silicon-based layered struc- be limited to low-power applications. remarkable promise for further en- ture rather than a hydrogenated Future Outlook. Despite drawbacks, hancement or molecular optimiza- silicon host structure. A wide variety the future of germanane itself may tion of the electronic and optical of Zintl phase materials exist for be limitless. Germanane is an exciting properties. Electrochemical interca- which topochemical transformation new 2D functionalized material with lation can be used to insert alkali has never been performed (Table 1 þ much promise, especially with the metals such as Li between few layers. suggests several). Potentially, this possibility of using covalent chemis- Organic intercalation, or the similar could give rise to a whole host of try to alter its optoelectronic or ma- zero-valent metal intercalation, may novel layered materials, which could terial properties. Hydrogen terminat- also be worth investigating.16 be directly exploited to make new 2D

KOSKI AND CUI VOL. XXX ’ NO. XX ’ 000–000 ’ XXXX D www.acsnano.org PERSPECTIVE monolayers with unique electronic, cells, sensors, high frequency cir- 12. Brandt, M. S.; Vogg, G.; Stutzmann, physical, and structural properties. cuits, and flexible electronics. M. Silicon-and Germanium-Based Sheet Polymers and Zintl Phases. In There are many examples of Zintl Conflict of Interest: The authors de- clare no competing financial interest. Silicon Chemistry. Jutzi, P., Schubert, phase alloy materials such as CaGaSi, U., Eds.; Wiley-VCH: New York, 2003; 17,18 pp 194213. CaZn2Sb2,andCa11GaSb9. Acknowledgment. We acknowledge 13. Hajnal, Z.; Vogg, G.; Meyer, L. J.-P.; Using a method similar to Bianco the support of two-dimensional materials Szücs, B.; Brandt, M. S.; Frauenheim, et al., combining topochemical research program by the Department of T. 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