Nature | Vol 577 | 16 January 2020 | 347found in Extended Data Tables 1, 2). LiInP 2 Se 6 comprises polyhedral
layers separated by van der Waals gaps. Scanning electron microscopy
images show a clear layered nature for this compound (Extended Data
Fig. 2c–d). Each individual layer has the structure of CdI 2 , where Li, In
and P 2 dimers each occupy one-third of the octahedral Cd sites in an
ordered manner and Se occupies the I site, as illustrated in Fig. 1a. The
stacking of these layers follows an ABAB sequence such that the InSe 6
polyhedra align whereas the LiSe 6 and P 2 Se 6 polyhedra alternate along
the c axis (Fig. 1b). This structure is typical among metal selenophos-
phates with the general formula M+M3+P 2 Se 6 (M, metal; refs.^18 ,^19 ) and
derives from a larger layered family based on divalent metal cations
M2+ 2 P 2 Se 6 (ref.^20 ). Raman spectroscopy at an excitation of 633 nm shows
several well-resolved peaks consistent with the vibrational modes of
the InSe 6 octahedra and the [P 2 Se 6 ]4− unit seen in Fig. 1d (see Extended
Data Fig. 3 for details).
The melting behaviour of LiInP 2 Se 6 was determined through dif-
ferential thermal analysis (DTA), giving melting and crystallization
points of 717 °C and 695 °C, respectively (Extended Data Fig. 2a). The
compound melts congruently, as a single thermal event is observed
upon heating or cooling, and the powder diffraction pattern after DTA
shows only LiInP 2 Se 6 present (Extended Data Fig. 1g).
We then developed a more effective approach to grow detector-
grade, large-size LiInP 2 Se 6 single crystals using the chemical vapour
transport (CVT) method. The temperature of the hot zone (660 °C)
was below the melting point because otherwise the atmosphere would
saturate with volatile components and inhibit the transport of the other
constituents. The cold zone was set to 560 °C, creating a driving force
for the transport of material from the hot to the cold zone. The initial
transport reactions of LiInP 2 Se 6 were carried out without a transport-
ing agent and produced tiny LiInP 2 Se 6 crystals with a low yield (about
1 mg out of ~0.7 g of starting material) even after a few days of transport
owing to decomposition into amorphous PxSey (Extended Data Fig. 1d).
This issue was resolved by the use of iodine as a transporting agent,
as the formation of InI 3 and LiI enables the transport of each metal
component to balance the vapour transport^21. Iodine-assisted CVT
enabled the growth of large single LiInP 2 Se 6 crystals with an area of
about 1 cm^2 and thickness of ~0.05–1 mm for the week-long reactions
in Fig. 1c. To reduce the number of nucleation sites and improve the
yield of large crystals, we reversed the zones for 24 h to ensure that all
LiInP 2 Se 6 was at the source side. The crystals produced by the transport
reactions without this first step are smaller and often intergrown with
each other owing to multiple nucleations (Extended Data Fig. 1e). The
reversal of the hot and cold sides at the end of the reaction limits the
deposition of the gas phase species onto the as-grown crystal surfaces,
which is necessary for high-quality, clean surfaces that permit good
electrical contact with the electrodes. The X-ray powder diffraction
pattern of ground CVT-grown LiInP 2 Se 6 crystals matches the simulated
pattern and exhibits identical thermal behaviour to the bulk material
(Extended Data Figs. 1f, h, 2b), confirming the purity of the transported
LiInP 2 Se 6 single crystals.
Density functional theory calculations of the electronic band struc-
ture were carried out using the PBE+vdW functional, revealing LiInP 2 Se 6
to be an indirect-gap semiconductor with a bandgap transition
from near the Κ point at the valence band maximum to the Γ point at
the conduction band minimum for a bandgap of 0.94 eV (Fig. 2a). The
conduction band is composed of hybridized In s and Se p states associ-
ated with the empty In 5s lone pair, whereas the valence band is made
up of Se p states, as seen in Fig. 2b. The conduction band has a high
curvature, corresponding to a low effective electron mass (me⁎) of 0.16me
and 0.30me for the in- and out-of-plane directions, respectively (me,
electron mass). The valence band maximum lies along the out-of-plane
direction in reciprocal space (K to H), and the band in this direction is
very flat, so the hole effective mass in the out-of-plane direction will
be immense. Accordingly, we expect that the electron mobility should
be vastly superior in LiInP 2 Se 6 , especially given that the planar nature
of the material forces devices to operate in the out-of-plane direction.
Ultraviolet–visible–near infrared (UV-Vis-NIR) transmission and
reflection measurements on a LiInP 2 Se 6 single crystal were used to
measure the bandgap, which was determined to be 2.06 eV (Fig. 2c).
Photoluminescence (PL) measurements on LiInP 2 Se 6 at low temperature
(12.5 K) under an excitation of 405 nm showed red–orange emission from
the as-grown surface of the single crystal, with two broad emission bands
observed with peak maxima near 1.73 eV and 2.05 eV (Fig. 2d). Excitation
intensity and temperature-dependent PL measurements support the
assignment of the 1.73 eV peak to donor–acceptor pair recombinationbbc aLi
In
P
Seacabc5.328 ÅdIntensity (a.u.)100 200 300 400 500
Wavenumber (cm–1)LiInP 2 Se 6
295 K, 25 mW
633 nm RamanP 2 Se 6 -likeP 2 Se 6 -likeIn-Se-likeFig. 1 | Structural properties of LiInP 2 Se 6. a, b, Crystal structure of LiInP 2 Se 6 : a
2 × 2 unit cell looking down the c axis (a) and a single unit cell looking down the
a–b plane, showing layered structure. c, Source side of the reaction vessel
containing plate-like crystals of CVT-grown LiInP 2 Se 6. d, Raman spectrum of
LiInP 2 Se 6 acquired at room temperature, showing characteristic vibrational
modes associated with In–Se bonds and P 2 Se 6 units. a.u., arbitrary units.
bE – EVBM (eV)–10 –5 05Density of states
(eV−1 per unit cell volume) 04812c dWavevectorΓΓMK ALHAaEEF(eV)40
–2
–42
CBM VBMSe s
Se p
P s
In sP p
In pb 1 b^2L
MKA
HΓb 360
50
40
30
20
10
0Transmission (%)500 750 1,0001,2501,500
Wavelength (nm)1/2D(cm–1/2)Energy (eV)Eg = 2.06 eVPL intensity (a.u.)1.4 1.6 1.8 2.0 2.2
Energy (eV)12.5 K, 2 mW PL
Cumulative t
1
2320
10
0
123LMKHFig. 2 | Band structure and optical properties of LiInP 2 Se 6 single crystals.
a, b, Electronic band structure (a) and density of states (number of states per
unit cell volume per electronvolt) (b) for LiInP 2 Se 6 calculated using the
PBE+vdW functional on a fully relaxed structure. The inset in b shows the
Brillouin zone. CBM, conduction band minimum; VBM, valence band
maximum; E, energy; EF, Fermi energy. c, UV-Vis-NIR transmission spectrum of
LiInP 2 Se 6. The inset shows the corresponding Tauc plot. α, absorbance; Eg,
bandgap energy. d, PL spectrum of LiInP 2 Se 6 obtained at 12.5 K and 2 mW, with
three Gaussian fit peaks. The inset shows an image of the PL emission.