Covalently-Controlled Properties By Design In Group IV Graphane ...

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Covalently-Controlled Properties by Design in Group IV Graphane
Analogues
Published as part of the Accounts of Chemical Research special issue “2D Nanomaterials beyond Graphene”.
Shishi Jiang,‡ Maxx Q. Arguilla,‡ Nicholas D. Cultrara,‡ and Joshua E. Goldberger*
Department of Chemistry and Biochemistry, The Ohio State University, Columbus, Ohio 43210, United States
CONSPECTUS: The isolation of graphene has sparked a renaissance in the study of twodimensional materials. This led to the discovery of new and unique phenomena such as extremely
high carrier mobility, thermal conductivity, and mechanical strength not observed in the parent
3D structure. While the emergence of these phenomena has spurred widespread interest in
graphene, the paradox between the high-mobility Fermi−Dirac electronic structure and the need
for a sizable band gap has challenged its application in traditional semiconductor devices. While
graphene is a fascinating and promising material, the limitation of its electronic structure has
inspired researchers to explore other 2D materials beyond graphene.
In this Account, we summarize our recent work on a new family of two-dimensional materials
based on sp3-hybridized group IV elements. Ligand-terminated Si, Ge, and Sn graphane analogues
are an emerging and unique class of two-dimensional materials that offer the potential to tailor the
structure, stability, and properties. Compared with bulk Si and Ge, a direct and larger band gap is
apparent in group IV graphane analogues depending on the surface ligand. These materials can be
synthesized in gram-scale quantities and in thin films via the topotactic deintercalation of layered Zintl phase precursors. Few
layers and single layers can be isolated via manual exfoliation and deintercalation of epitaxially grown Zintl phases on Si/Ge
substrates. The presence of a fourth bond on the surface of the layers allows various surface ligand termination with different
organic functional groups achieved via conventional soft chemical routes. In these single-atom thick materials, the electronic
structure can be systematically controlled by varying the identities of the main group elements and by attaching different surface
terminating ligands. In contrast to transition metal dichalcogenides, the weaker interlayer interaction allows the direct band gap
single layer properties such as photoluminescence to be readily observable without the need to exfoliate down to single layers.
Furthermore, these materials can be resilient to oxidation and thermal degradation, making them attractive candidates for next
generation functional materials for electronic devices and beyond.
This class of two-dimensional materials not only are promising building blocks for a variety of conventional semiconductor
applications but also provide a pioneering platform to systematically and rationally control material properties using covalent
chemistry. The stability and tunability of these versatile materials will push this system toward the forefront of two-dimensional
research.
■ INTRODUCTION
As a result of the widespread integration of semiconductor
technology into all facets of life, the group IV semiconductors,
silicon and germanium, are the most important and ubiquitous
materials of the current era. Not only are they the workhorse
materials of transistor technology, silicon and germanium are
the most prevalent materials employed in photovoltaics1 and
photodetectors2 and have attracted considerable attention as
thermoelectric energy generators.3,4 Still, the neverending push
toward device miniaturization calls for the need to understand
the nature of these materials when reduced below the
nanoscale. The creation of single-atom thick layers provides
an avenue for the discovery of new phenomena and properties
that can potentially overcome some inherent limitations in the
parent three-dimensional (3D) semiconductors. For example,
the indirect nature of silicon and germanium’s band gap limits
their efficiency in optoelectronics, and prevents their
implementation into light emitting applications.
Graphene’s discovery5 has shown that it is possible to
prepare single atom thick layers of a two-dimensional (2D)
material, and as a consequence, numerous methods6 have been
developed to facilitate the understanding of the unique
properties that emerge in single layers. Graphene is a single
layer of graphite and is a comprised of a π-bonded honeycomb
lattice of carbon atoms. Graphene, with its linear dispersion at
the K point and massless Dirac fermions, has unique properties
like high carrier mobilites (∼200 000 cm2 V−1 s−1), leading to
the observation of the quantum hall effect at room temperature,
as well as high thermal conductivity and exceptional mechanical
strength.7−9 Nevertheless, the fact that this high mobility state
only appears as a result of the linear Fermi−Dirac dispersion of
carbon’s half-filled 2pz orbitals and semimetallic zero band gap
Special Issue: 2D Nanomaterials beyond Graphene
Received: August 11, 2014
Published: December 9, 2014
Article
pubs.acs.org/accounts
© 2014 American Chemical Society 144 dx.doi.org/10.1021/ar500296e | Acc. Chem. Res. 2015, 48, 144−151
limits the ability to readily integrate graphene into current
semiconductor technology, which requires materials with band
gaps for optimal performance. For example, the lack of a band
gap prevents graphene transistors from having large ratios in
current between the on and off state. Functionalization of
graphene to make hydrogen-terminated graphene, or graphane,
opens a sizable band gap but dramatically decreases the carrier
mobility to 10 cm2 V−1 s−1, by bonding to the C 2pz orbitals
thereby eliminating the Fermi−Dirac state.10 While graphene is
a fascinating and promising material, the limitations of its
electronic structure has inspired researchers to explore other
2D materials beyond graphene.
An entire field of research has emerged investigating other
similar 2D van der Waals solids.6 These materials allow for
manual exfoliation to single and few-layers, breaking the weak
interlayer interactions while maintaining the strong in-plane
bonding. While there is an ever expanding class of these
materials, this Account will primarily focus on the group IV
(silicon, germanium, and tin) analogues of graphane. These
structures are comprised of 2D puckered honeycomb networks
of sp3-hybridized group IV atoms and are terminated with
hydrogen or other ligands (Figure 1).
Although they all belong to group IV, heavier elements like
Si, Ge, and Sn do not readily form π-bonds. This arises from
their larger atomic size, which increases their bond distances,
thereby reducing overlap between nearest neighbor π-bonding
p orbitals. In other words, each Si, Ge, or Sn atom would
preferentially bond to another ligand rather than form a π-bond
with its neighbor. Since every atom in the 2D network has a
covalently bound ligand, the identity of this ligand can provide
a versatile synthetic handle for tuning the electronic structure
and properties in these materials. For example, with the
appropriate surface terminating ligand, these 2D materials can
feature direct band gaps,11−15 potentially enhancing silicon’s
and germanium’s performance in photovoltaics, photodetectors, light-emitting diodes, and lasers. Furthermore, these
puckered honeycomb networks are structurally analogous to Si
and Ge(111) surfaces, allowing the use of established surface
functionalization chemistries16−20 to modify the ligand.
In this Account, we highlight the various routes toward
synthesizing these group IV graphane analogues. We show that
this system can be covalently modified with various organic
functional groups. We also discuss the influence of the surfaceterminating ligand and the main group element on the
electronic structure and stability of these 2D materials.
■ TOPOTACTIC SYNTHESIS
Though the synthesis of polysilanes, polygermanes, and
polystannanes from small molecules has been well developed,21−23 they are limited to 1D inorganic polymers and single
rings. To date, there have been no synthetic routes for
preparing group IV graphane analogues from small molecule
precursors due to the lack of a mechanism for controlling
growth in two dimensions. However, the layered Zintl phases,
CaSi2, CaGe2, and BaSn2, are comprised of puckered
honeycomb [Si−]n, [Ge
−]n, and [Sn
−]n graphane-like layers
held together by the group II [M2+] cations (Figure 2a).
Consequently, the preparation of group IV graphane analogues
relies on developing soft chemical processes that can
topotactically deintercalate the M2+ cations while maintaining
the structure and covalently terminating the anionic group IV
layers.
The topotactic deintercalation of CaSi2 using HCl can be
traced back to Wöhler in the 1860s and Kautsky in the 1920s,
and the structure and properties were partially resolved in the
1980s and 1990s.11,12,24−29 It was reported that siloxene
(Si6H3(OH)3) preferentially forms at temperatures greater than
0 °C and layered polysilane (Si6H6−x(OH)1−x (x < 1)) forms at
−30 °C.11,12,27 These structures are silicon graphane analogues,
or silicanes, with −H and −OH or mostly −H terminal
substituents, respectively. Compared with the indirect band gap
of crystalline silicon (1.1 eV), Siloxene has a direct band gap at
Figure 1. Model of GeH. View from (a) the (100) and (b) the (001)
directions. (Ge, blue; H, black).
Figure 2. Schematic illustration of topotactic deintercalation of (a)
CaGe2 to (b) GeH (Ca, yellow; Ge, purple; H, black). Optical images
of (c) CaGe2 and (d) GeH crystals with select crystals on a 1 mm grid
graph paper. Powder XRD of (e) CaGe2 and (f) GeH.
13
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around 2.4 eV with strong photoluminescence (PL).12 Other
silicon graphane analogues terminated with organic functional
groups were also reported in the past decade that feature PL
ranging from 2.7 to 2.9 eV.30−32 However, all these reactions
rely on the topotactic deintercalation of CaSi2 in aqueous HCl,
which readily produces partially OH-terminated SiHx(OH)1−x
due to the significantly stronger Si−O bond (800 kJ/mol)
compared with the Si−H bond (300 kJ/mol). This ambiguity in
surface functionalization convolutes efforts to correlate the
effects of surface functionalization on the optoelectronic
properties of these single-atom thick semiconductors. In
contrast, the difference of bond strength is much smaller
between Ge−O (660 kJ/mol) and Ge−H (320 kJ/mol) and
furthermore, any native germanium oxide or hydroxide
termination is readily dissolved in aqueous HCl, thereby
producing pure germanane (GeH). Indeed it was reported by
Brandt and Stutzmann that CaGe2 thin films grown on
germanium wafers can be topotactically deintercalated to form
GeH, with little surface oxidation.33
Recently our group has synthesized for the first time
millimeter-scale crystals of GeH via the topotactic deintercalation of large CaGe2 single crystals in aqueous HCl (Figure
2).13 Here, Ca2+ is removed via the formation of a soluble
CaCl2 species and the anionic [Ge
−]n layer is terminated by H
atoms. X-ray diffraction (XRD) confirms that the layered
hexagonal germanium lattice is maintained, and an increase in
the interlayer distance occurs (5.1 to 5.5 Å) upon replacing the
Ca2+ with two Ge−H bonds (Figure 2e,f). The large full-width
at half-maximum of all the peaks that contain any c-axis
reflections indicates that there exists a significant amount of
disorder in the c-axis, which is common in layered materials.
Pair distribution function (PDF) analysis collected from
synchrotron measurements directly confirms the honeycomb
2D network of germanium atoms.34 Compared with the PDF of
crystalline Ge, GeH has systematic absences at 5.66, 7.35, and
8.95 Å (Figure 3b) that arise in 3D crystalline Ge. In Ge, these
peaks correspond to Ge−Ge pairs between atoms in different
(111) layers. All other peaks can be indexed to the Ge−Ge
pairs within a single Ge(111) plane. The interlayer disorder of
GeH prevents the observation of scattering between any
interlayer Ge−Ge pairs. Furthermore, transmission electron
microscopy (TEM) analysis indicates the layered morphology
of GeH (Figure 3c,d). Finally, the most conclusive technique
for determining the nature of the ligand bonded to each
germanium atom is Fourier transform infrared spectroscopy
(FTIR). Every vibrational mode observed in the FTIR
corresponds to a Ge−H bond, with no peaks corresponding
to any Ge−O vibrational mode. The identity of each mode was
readily verified with deuterium labeling.13
The deintercalation of Zintl phases that contain multiple
group IV elements enables the synthesis of alloy graphane
analogues. We have been able to substitute up to 9% of the Ge
atoms with Sn (Figure 4a) in the precursor Zintl phase
(CaGe2−2xSn2x).
35 Deintercalation in aqueous HCl produces a
2D honeycomb network where tin is OH-terminated while
germanium remains H-terminated (Figure 4b). Raman spectroscopy confirms that alloying occurs, because the shifts in
both the in-plane phonon (E2) and cross-plane (A1) vibrational
modes are consistent with the expected differences based on
the changes in the reduced mass (Figure 4c).
Single and Few Layer Thick Materials
The synthesis of large millimiter-scale flakes of GeH enables the
isolation of single and few layer thick sheets via mechanical
exfoliation using polydimethylsiloxane (PDMS) and Scotch
tape (Figure 5a), by adapting the procedure developed by
Frindt.36,37 These single-layer flakes were exfoliated onto 110
nm thick and 300 nm thick SiO2/Si substrates, which provide
suitable optical contrast. However, mechanical exfoliation is a
labor-intensive and nonscalable process for producing single
and few-layer thick materials. Additionally, the interlayer van
der Waals interactions in GeH are calculated to be ∼35%
stronger in GeH compared with graphene,13 which experimentally makes the preparation of large areas of single layer
GeH more challenging. The ability to synthesize precise layer
thicknesses of these group IV graphane analogues on Si and Ge
Figure 3. (a) A single (111) plane of crystalline germanium,
representing a single layer of GeH. The distance of the germanium
atoms on a certain colored ring from the central germanium atom
corresponds to the same color peak in panel b. (b) PDFs of GeH and
Ge. The starred peaks correspond to the interactions between
germanium atoms in different layers.34 (c) Low-magnification and
(d) magnified TEM micrograph of GeH platelets. Inset in panel c is
the corresponding electron diffraction pattern collected down the
[001] zone axis.13
Figure 4. (a) Topotactic deintercalation of CaGe2−2xSn2x to
Ge1−xSnxH1−x(OH)x in HCl (Ca, yellow; Ge, blue; H, black; O, red;
Sn, green). (b) FTIR and (c) Raman spectra of Ge1−xSnxH1−x(OH)x
(x = 0−0.09).35
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substrates, would enable their seamless integration into existing
semiconductor fabrication infrastructure. This can be achieved
by first epitaxially growing the precursor Zintl phases onto Si
and Ge substrates. The a,b-parameters of the precursor CaGe2
Zintl phase closely match the spacing of the Ge(111) surface to
less than 0.5%. This enables the direct epitaxial growth of
CaGe2 thin films on Ge(111) wafers, which can be
subsequently topotactically deintercalated to obtain GeH.
Indeed, we have prepared 5 nm thick films of CaGe2 on
Ge(111) via molecular beam epitaxy (MBE), which would
correspond to ∼10 layers.38 These co-deposited CaGe2 thin
films have grain sizes on the order of a few micrometers, which
is the typical size of terrace formed due to the miscut of the
Ge(111) growth substrate. Upon treatment in HCl, these thin
films exhibit the same XRD and Raman profiles as those
produce from single crystals of CaGe2. Consequently, the
combination of epitaxial growth and topotactic deintercalation
represents a promising and scalable route for the preparation of
precise layer films of group IV graphane analogues and
simplifies subsequent very large scale integration (VLSI)
processing.
■ COVALENTLY MODIFIABLE BUILDING BLOCKS
The presence of a covalently bound surface ligand on every
atom in these group IV graphane analogues opens up the
possibility of tuning the properties by varying this surface
ligand. There has been extensive work during the past few
decades showing that every atom on Si and Ge(111) surfaces
can be terminated with small organic substituents such as
−CH3 and −CCH.16−20,39 In contrast to H-terminated Si/
Ge(111) surfaces, which oxidize within 30 min of exposure to
air, these organic-terminated surfaces have been shown to be
resistant toward oxidation for at least 30 days.19,39,40
Consequently, it is easy to envision 2D derivatives of these
same organic functionalized surfaces.
To these ends, we have developed a one-step metathesis
approach that directly converts CaGe2 crystals into organicterminated germananes by topotactically reacting them with
organoiodines. For instance, we have prepared ∼1 mm flakes of
GeCH3 (Figure 6a,b) by reacting CaGe2 with CH3I.
14 Through
this reaction, Ge− anions bond to the CH3 group, and the
iodide reacts with Ca2+ to form a soluble CaI2 species, which is
easily separated. Single crystal and powder XRD analysis show
that the hexagonal unit cell of CaGe2 is retained and GeCH3
has a similar 2H unit cell (two GeCH3 layers per unit cell) as
GeH (Figure 6c,d). The interlayer distance of GeCH3 is
increased by 3.1 Å compared with GeH, which is close to the
estimated increase (∼2.5 Å) based on the bond length and van
der Waals radii differences of these two ligands. The methyl
termination is further confirmed by FTIR measurements
(Figure 6e). Compared with spectra of GeH, the intense
Ge−H stretching mode at 2000 cm−1 is almost entirely gone,
while a Ge−C stretching mode at 573 cm−1 is observed. Other
vibrational modes like −CH3 stretching, bending, and rocking
modes are also detected. The identity of each mode can be
further verified upon comparison with the FTIR spectra of
Ge13CH3 and GeCD3.
This one-step metathesis method is a general route for
preparation of organic ligand terminated germananes. By
substituting CH3I with other organoiodine reagents like
CH3CH2I and CH2CHCH2I, we have prepared CH3CH2Ge
and CH2CHCH2Ge, respectively. The interlayer spacing is
expected to increase by 3.5 and 6.2 Å, upon replacing −H in
GeH with −CH2CH3 and −CH2CHCH2, respectively. This
is in close agreement with the increases in interlayer spacing of
3.7 and 5.8 Å observed via XRD (Figure 7a). In FTIR spectra,
Ge−C stretching can be detected in all three spectra along with
the near elimination of the Ge−H stretching mode. All the
other vibrational modes can be assigned to the corresponding
Figure 5. AFM images of (a) an exfoliated single layer of GeH and (b)
GeH thin film after deintercalation of a 5 nm thick CaGe2 film grown
via MBE on Ge(111). Inset in panel a is an optical micrograph of the
single-layer flake.13,38
Figure 6. (a) Model of GeCH3 (Ge, blue; C, black; H, gray). (b)
Optical images of GeCH3 crystals with select crystals on 1 mm grid
graph paper. (c) Single-crystal XRD pattern of GeCH3 collected down
the [001] zone axis. (d) Powder XRD pattern of GeH and GeCH3.
The starred peaks correspond to diffraction reflections of an internal
Ge standard. (e) FTIR spectra of GeH, GeCH3, Ge
13CH3, and
GeCD3. The intensities of the four spectra are multiplied by 0.5 from
400 to 900 cm−1.14
Figure 7. (a) Powder XRD pattern and (b) FTIR spectra of GeCH3
(black), GeCH2CH3 (blue), and GeCH2CHCH2 (red).
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organic functional groups. The observation of H−CC−
stretching and bending modes at 3076 and 1626 cm−1 further
confirms the termination with −CH2CHCH2. The versatility
of this reaction scheme enables the grafting of functional
ligands with tunable polarity, reactivity, and mechanical
strength for a wide variety of applications.
■ TUNING THE ELECTRONIC STRUCTURE
The rich surface functionalization chemistry allows these group
IV graphane analogues to be highly tunable electronic and
optoelectronic building blocks for next generation devices. By
substituting the main group element and varying the surface
ligand, one can tune the electronic structure of these materials
to produce unique properties that do not exist in the parent 3D
semiconductor structure. To understand how the presence of a
surface ligand influences the electronic structure of these 2D
graphane analogues, here we describe density functional theory
(DFT) simulations illustrating the difference between silicene
and silicane as a model system.41,42
Silicene is a 2D material comprised of a honeycomb
arrangement of Si atoms in which every Si shares three σ and
one π bond with the three neighboring Si atoms.42,43 Similar to
graphene, this structure exhibits Fermi−Dirac behavior at the K
point on account of the half-filled 3pz orbitals (Figure 8a).
Adding hydrogen as a surface terminating ligand to silicene
produces silicane (SiH) through the formation of a covalent
bond with the Si 3pz orbital.
42−44 This bonding and
antibonding interactions splits the Dirac cone at the K point
thus opening a sizable band gap (Figure 8b). The electronic
band structure of silicane calculated at the HSE-0645,46 level
predicts an indirect band gap of 2.94 eV from Γ to M and a
direct band gap of 3.14 eV at Γ.42 The conduction band valley
at Γ is comprised of Si−H σ* states, the conduction band
minimum (CBM) at M corresponds to Si−Si σ* states, whereas
the valence band maximum (VBM) corresponds to Si−Si σ
states (Figure 8b).
The band structures of GeH and SnH are closely related to
SiH.42,43 However, in the case of GeH, the CBM occurs at Γ
and not M, leading to a 1.56 eV direct band gap with an
effective electron mass of me,Γ* = 0.09. This is consistent with
the observed absorption onset at 1.59 eV and with the
observation of PL in GeH at 1.56 eV at low temperature. The
direct band gap of GeH is in sharp contrast to the 0.67 eV
indirect bap gap of crystalline germanium.47 In crystalline
germanium, the CBM occurs in the four equivalent valleys at
the L ⟨111⟩ point, which has a much higher effective mass
(me,L* = 1.64) than the conduction band valleys at Γ (me,Γ* =
0.041). However, since GeH can be thought of as hydrogenterminated isolated (111) sheets of germanium, we are
effectively eliminating the L wavevector in the Ge Brillouin
zone, resulting in a material that has a direct gap and a
considerably higher electron mobility. Electron mobility is
inversely proportional to effective mass. We calculated from
first-principles the phonon-limited electronic mobility for an
isolated single layer of GeH obtaining a room temperature
mobility of ∼18000 cm2 V−1 s−1. This 5× increase in electron
mobility from bulk Ge (3900 cm2 V−1 s−1) is consistent with
the reduced electron effective mass in GeH. These group IV
graphane analogues also feature significantly larger band gaps
compared with the parent 3D material (Table 1).
The 2D sp3-hybridized Sn is predicted to be a topological
insulator when terminated by electronegative groups such as
−OH and various halides, yet remains as a trivial insulator
when terminated by smaller less electronegative ligands such as
−H.15 This topological phase emerges when the Sn 5s σ* bands
drop below the 4px and 4py VBM, which is split on account of
the large spin−orbit coupling in Sn. Consequently, 2D tin is
predicted to exhibit the quantum spin Hall effect.
One way to tune the electronic structure of these 2D
materials is through alloying different group IV elements in the
framework. We have systematically tuned the band gap of GeH
from 1.59 eV down to 1.38 eV (Figure 10a,b) through alloying
up to 9% Sn into the CaGe2−2xSn2x lattice and topotactically
deintercalating the lattice with aqueous HCl, based on of diffuse
reflectance absorption (DRA) measurements.35 Increasing the
Sn percentage in the lattice can potentially lead to band gaps
Figure 8. DFT simulations of the electronic band structure of (a)
silicene and (b) silicane and the density orbitals at the high symmetry
k-points: (A) Si−H σ*, (B) Si−Si σ*, (C, D) Si−Si σ states. Inset in
panel a is the hexagonal Brillouin zone.42
Table 1. Band Gaps of Different sp3-Hybridized Group IV
Elements in Bulk and in 2D
3D sp3 (eV) 2D sp3 (eV)
C 5.48 (indirect)48 3.5a (−H, direct)49
Si 1.12 (indirect)50 2.4 (−OH/−H, direct),11 2.94a (−H,
indirect)42
Ge 0.67 (indirect)47 1.59 (−H, direct)13 1.71 (−CH3, direct)14
Sn 0 ± 0.05 (β-, direct)51 ∼0.30a (−H, −halides, direct)15
aTheoretical.
Figure 9. Electronic band structure of an isolated single layer of GeH
calculated using HSE-06 theory including spin−orbit coupling
predicting a 1.56 eV direct band gap. The hole and electron effective
masses for each extrema are indicated in red.13
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down to 0.3 eV, which would open up their application as
tunable photodetectors suitable for telecommunications.
Aside from alloying the main group element, another route
that can be used to tune the properties of these materials is
through the variation of the surface ligand. Because the CBM at
Γ corresponds to the group IV−ligand σ* antibond, this energy
level can be raised or lowered depending on the electronegativity of the surface terminating ligand. Since the ligand
also influences the geometry of the entire lattice, this also leads
to changes in the bond lengths and bond angles in the 2D
network, significantly affecting the rest of the band structure.
Therefore, substituting one ligand for another will affect the
entire electronic band structure and can significantly change the
observed band gap and direct/indirect nature. A systematic
experimental study comparing the electronic and optical
properties of uniformly functionalized 2D group IV graphane
analogues is needed to foster a deeper understanding of the
influence of surface ligands on the electronic structure of these
materials.
Experimentally, we have demonstrated that the band gap of
GeH can be increased by 0.1 eV (Figure 10c) by replacing the
−H with −CH3.
14 Furthermore, very intense band edge PL is
observed in GeCH3 (Figure 10d) with a quantum yield of 0.2%.
In contrast to other layered materials like MoS2, this PL is
independent of layer thickness, thus obviating the need for
large area single layers for practical devices. The change in PL
indicates that optical properties of these 2D group IV graphane
analogues can potentially be improved with different ligand
terminations. In summary, the combination of alloying and
covalent functionalization provides an unprecedented level of
control over electronic structure in a 2D crystal, creating a
versatile platform for electronic and optoelectronic applications.
■ THERMAL AND AIR STABILITY
The potential utility of these group IV graphane analogues for
any functional device strongly hinges on their air and
temperature stability. It has been established that siliconbased 2D systems such as SiH readily oxidize upon exposure to
air.31,44 GeH, however, shows a remarkable resistance to
oxidation.13 The lack of oxidation in the bulk material is
observed in FTIR measurements (Figure 11a) after air
exposure of up to 60 days where the absence of different
GeOx vibrational modes
52 from 800 to 1000 cm−1 indicate that
the bulk of GeH is unchanged. On the other hand, X-ray
photoelectron spectroscopy (XPS) (Figure 11b) is the most
sensitive technique in determining the presence of oxidation on
the surface. After five months of exposure to air, a Ge2+/3+
shoulder emerges at 1219.3 eV (29.7% Ge2+/3+) indicating that
surface oxidation is prevalent. After Ar ion etching of the top
0.5 nm (∼1 layer), the Ge2+/3+ peak almost completely
disappears with 10.1% Ge2+/3+ remaining.13 Together, the
XPS and FTIR mothers suggest that GeH is resilient toward
oxidation and only the surface layer slowly oxidizes.
The thermal stability of these 2D materials strongly depend
on the identity of the surface terminating ligand. We observed
that in GeH, the emergence of thermal-induced amorphization
begins at 75 °C.13 We have observed that measurements
focused on changes in properties are the most sensitive
methods for detecting amorphization, compared with bulk
structural analyses like XRD and Raman spectroscopy. The
absorption of the new and emerging amorphous germanium
species lower than the 2D germanane band gap is more readily
observable than the gradual disappearance of a crystalline phase
in XRD or Raman. Upon annealing at 75 °C and above, there is
an increasing red shift in the absorption onset, consistent with
the formation of amorphous GeH and, at higher temperatures,
amorphous Ge (Figure 11c).13 This amorphization process was
further confirmed with temperature dependent PDF measurements.34 In contrast, GeCH3 starts to amorphize at a
significantly higher annealing temperature, at around 250 °C
(Figure 11d).14 In summary, the oxidation resistance of
germanane and the improvement in thermal stability by methyl
Figure 10. (a) DRA spectra of Ge1−xSnxH1−x(OH)x (x = 0 to 0.09)
plotted in terms of the Kubelka−Munk function illustrating the
consistent red shift in the absorption onset.35 (b) Sn-dependent
optical band gap of Ge1−xSnxH1−x(OH)x alloys obtained via a linear
approximation of the absorption edge.35 (c) DRA spectra of GeH
compared with GeCH3 and (d) absorption and PL spectra of GeCH3
with the actual PL observed in isopropyl alcohol as an inset.14
Figure 11. (a) FTIR spectra of GeH after exposure to air for up to 60
days highlighting the absence of any Ge−O vibrational mode. (b) XPS
spectra of GeH exposed to air for up to five months, followed by 0.5
nm Ar etch. DRA spectra of (c) GeH and (d) GeCH3 annealed in Ar/
H2 gas at various temperatures.
13,14
Accounts of Chemical Research Article
dx.doi.org/10.1021/ar500296e | Acc. Chem. Res. 2015, 48, 144−151149
termination makes these materials attractive candidates for next
generation devices and can be potentially robust enough to
withstand the demands for fabrication and operation.
■ CONCLUSION AND OUTLOOK
These group IV graphane analogues are real materials that can
be synthesized as robust single crystals in gram-scale quantities,
exfoliated into single layers, and prepared on conventional
VLSI substrates as few-layer thin films. Compared with all other
2D van der Waals solids, these group IV graphane analogues
offer the capability for covalent surface termination, providing a
versatile handle for tailoring the structure, stability, and
electronic properties in single-atom-thick materials. Such
exquisite control over the material properties makes these
systems attractive candidates for a multitude of applications
such as sensing, optoelectronics, and thermoelectrics and also
offers the potential for new physical phenomena such as the
quantum spin Hall effect. Leveraging previously established
semiconductor surface functionalization routes will enable the
creation of entire libraries of organic ligand terminated 2D
materials. Overall, this new class of 2D materials is posed to
have a great impact not only in the traditional sectors of
nanoscience but also in opening up a new research paradigm in
covalently controlled properties by design.
■ AUTHOR INFORMATION
Corresponding Author
*E-mail: goldberger@chemistry.ohio-state.edu.
Author Contributions
‡S.J., M.Q.A., and N.D.C. contributed equally.
Notes
The authors declare no competing financial interest.
Biographies
Shishi Jiang received her B.S. (2011) in chemistry from University of
Science and Technology of China. She is currently working towards
her Ph.D. in Chemistry from The Ohio State University. Her research
focuses on the covalent functionalization of group IV graphane
analogues.
Maxx Arguilla received his B.S. degree from the University of the
Philippines-Diliman in 2011. He is currently a graduate student in the
Chemistry department of The Ohio State University. His research
mainly focuses on germanium/tin alloys and tin-based group IV
graphane analogues.
Nicholas Cultrara received a B.S. from State University of New York,
University at Buffalo, in 2012 and is currently working towards his
Ph.D. in Chemistry at The Ohio State University. His research
primarily focuses on the doping and electronic characterization of
group IV graphane analogues.
Joshua Goldberger received his B.S. from The Ohio State University
in 2001 and his Ph.D. in Chemistry from University of California,
Berkeley, in 2006. He did postdoctoral research at Northwestern
University before joining the faculty in the Department of Chemistry
at The Ohio State University in 2010. His main research interests
focus on solid-state materials at the atomic scale, and their applications
in electronics, energy conversion, and photonics. More information
about the Goldberger group can be found at http://research.
chemistry.ohio-state.edu/goldberger/
■ ACKNOWLEDGMENTS
We graciously acknowledge Wolfgang Windl for DFT
simulations. The materials development was supported by the
Army Research Office (Grant W911-NF-12-1-0481), and the
simulations were supported by the Center for Emergent
Materials at The Ohio State University, an NSF MRSEC center
(Grants DMR-0820414 and DMR-1420451).
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