F. Kargar et al Specifics of the Elemental Excitations in True -1D MoI 3 van der Waals Nanowires 2022 1 P a g e Specifics of the Elemental Excitations in True -1D MoI 3 van der

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F. Kargar et al, Specifics of the Elemental Excitations in “True-1D” MoI3 van der Waals Nanowires (2022)

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Specifics of the Elemental Excitations in “True-1D” MoI3 van der

Waals Nanowires

Fariborz Kargar1,*, Zahra Barani1, Nicholas R. Sesing2, Thuc T. Mai3, Topojit Debnath4, Huairuo

Zhang5,6, Yuhang Liu4, Yanbing Zhu7, Subhajit Ghosh1, Adam J. Biacchi8, Felipe H. da Jornada9,

Ludwig Bartels10, Tehseen Adel3, Angela R. Hight Walker3, Albert V. Davydov6, Tina T. Sal-

guero2, Roger K. Lake4, and Alexander A. Balandin1,

1Nano-Device Laboratory (NDL) and Phonon Optimized Engineered Materials (POEM) Center,

Department of Electrical and Computer Engineering, University of California, Riverside,

California 92521 USA

2Department of Chemistry, University of Georgia, Athens, Georgia 30602 USA

3Quantum Measurement Division, Physical Measurement Laboratory, National Institute of

Standards and Technology (NIST), Gaithersburg, Maryland 20899 USA

4Laboratory for Terascale and Terahertz Electronics (LATTE), Department of Electrical and

Computer Engineering, University of California, Riverside, California 92521 USA

5 Theiss Research, Inc., La Jolla, California 92037, USA

6Materials Science and Engineering Division, National Institute of Standards and Technology,

Gaithersburg, Maryland 20899 USA

7Department of Applied Physics, Stanford University, Stanford, California 94305 USA

8Nanoscale Device Characterization Division, Physical Measurement Laboratory, National Insti-

tute of Standards and Technology (NIST), Gaithersburg, Maryland 20899 USA

9Department of Materials Science and Engineering, Stanford University, Stanford, California

94305 USA

10Department of Chemistry, University of California, Riverside, California 92521 USA

 Corresponding authors: fkargar@ece.ucr.edu (F.K.); balandin@ece.ucr.edu (A.A.B); web-site:

http://balandingroup.ucr.edu/

F. Kargar et al, Specifics of the Elemental Excitations in “True-1D” MoI3 van der Waals Nanowires (2022)

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Abstract

We report on the temperature evolution of the polarization-dependent Raman spectrum of exfoli-

ated MoI3, a van der Waals material with a “true one-dimensional” crystal structure that can be

exfoliated to individual atomic chains. The temperature evolution of several Raman features re-

veals anomalous behavior suggesting a phase transition of a magnetic origin. Theoretical consid-

erations indicate that MoI3 is an easy-plane antiferromagnet with alternating spins along the di-

merized chains and with inter-chain helical spin ordering. The calculated frequencies of the pho-

nons and magnons are consistent with the interpretation of the experimental Raman data. The ob-

tained results shed light on the specifics of the phononic and magnonic states in MoI3 and provide

a strong motivation for future study of this unique material with potential for spintronic device

applications.

Keywords: One-dimensional materials; van der Waals materials; quantum materials; antiferro-

magnetic materials; helical spin ordering; Raman spectroscopy; magnetic phase transitions

F. Kargar et al, Specifics of the Elemental Excitations in “True-1D” MoI3 van der Waals Nanowires (2022)

3 | P a g e

Recently a new research field focused on one-dimensional (1D) van der Waals (vdW) quantum

materials has emerged from the earlier work on low-dimensional systems.1–4 The 1D vdW mate-

rials are based on 1D structural motifs and include transition metal trichalcogenides and halides.1–

5 It is helpful to distinguish true-1D versus quasi-1D vdW systems. We define the material to be

true-1D if it contains covalent bonds only in the direction of the atomic chains, with all other

bonds being of vdW type; the material is quasi-1D if it contains strong covalent bonds along the

1D chain direction, while also bonded by weaker covalent bonds in the perpendicular planes. For

example, according to this criterion, Nb2Se9 is a true-1D material whereas TaSe3 is a quasi-1D

material.6–8 The number of vdW materials with quasi-1D or true-1D structures is not known, but

machine learning investigations indicate that hundreds of 1D vdW materials are synthetically

accessible.5,9–11 True-1D vdW materials can be exfoliated chemically or mechanically into indi-

vidual atomic chains or a few-atomic chain bundles.2,6,12 One can also envision that progress in

chemical vapor deposition (CVD) will eventually allow the controlled synthesis of such 1D ma-

terials on substrates of choice, thereby enabling their practical applications.13–15

Owning to their unique dimensionality and electronic density of states, 1D materials often support

unusual quasiparticle excitations, such as separated spin-charge excitations, Luttinger plasmons

with quantized velocities,16 magnetism,17 charge density waves, and emergent topology18–21.

Many of such unusual electronic properties also manifest themselves as nontrivial features in the

dispersion relation of other collective excitations, such as phonons and magnons, due to the lower

spatial dimensionality and stronger electronic correlation in 1D. The magnetic properties of 1D

materials are of particular interest. The intrinsic ferromagnetic (FM) and antiferromagnetic

(AFM) spin orderings have been demonstrated in the single atomic layers of some transition-

metal dichalcogenides (MX2) and transition-metal phospho-trichalcogenides (MPX3) that consti-

tute quasi-two-dimensional (2D) systems.22,23 In these compositions, “M” is a transition metal,

“X” is a chalcogen, and “P” is phosphorous. The possibility of AFM ordering in true-1D vdW

materials is interesting from both fundamental science and practical applications points of view.

The prospect of the helical spin order, i.e., spin-spiral order, in 1D materials is particularly excit-

ing.24 The challenges with the experimental observation of magnetism in the 1D limit are complex

owing to the scarcity of stable true-1D vdW material systems, low signal-to-noise ratio in mag-

netometry and other related experiments, and difficulties in handling individual atomic chains.

F. Kargar et al, Specifics of the Elemental Excitations in “True-1D” MoI3 van der Waals Nanowires (2022)

4 | P a g e

As a step in this direction, we investigated elemental excitations in exfoliated van der Waals

nanowires of MoI3, classified as a true-1D vdW material, using Raman spectroscopy.12 The latter

can be useful for spintronic applications; one can envision the atomic chains of the MoI3 serving

as ultimately downscaled magnonic interconnects conducting spin currents.25

MoI3 crystals for this study were prepared from the elements using the chemical vapor transport

(CVT) technique.12,26 The inclusion of NH4I as a transport agent led to a more reliable synthetic

procedure, owing to lower internal ampule pressures, and excellent product quality, which al-

lowed us to determine the single crystal structure of MoI3 by x-ray diffraction; details and atomic

coordinates are provided in the Supplemental Materials. Figure 1 (a) presents the crystal structure

of MoI3 from different views. Although this structure can be refined in both Pmmn and P63/mcm

with low R values, we ultimately used Pmmn in line with the interpretation of powder x-ray dif-

fraction (PXRD) data by Meyer and coworkers.26 The Pmmn structure was further confirmed by

our high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM)

imaging. The choice of space group is critical because Pmmn leads to “dimerized” MoI3 chains

showing clear Peierls distortion with alternating Mo–Mo distances of 2.8793(19) and 3.5298(19)

Å, whereas P63/mcm leads to 1D chains of equidistant Mo atoms. We also validated the space

group by PXRD; Figure 1 (b) shows the experimental pattern from MoI3 crystals with selected

hkl indices (see Supplementary Information for the complete hkl assignment). Refined lattice pa-

rameters from the single crystal study [a = 12.3183(4) Å, b = 6.4091(2) Å, c = 7.1180(3) Å] are

similar to those from our PXRD analysis [𝑎 = 12.3200(6) Å, 𝑏 = 6.4088(6) Å, 𝑐 =

7.1225(3) Å] as well as literature values.26 Figure 1 (c, d) presents the low-magnification

HAADF-STEM and atomic-resolved image of the exfoliated MoI3 crystals. The atomic-resolved

model of MoI3 confirms the Pmmn lattice structure along the [100] projection. Note that the scan-

ning electron microscopy (SEM) image of representative MoI3 crystals, presented in Figure 1 (e),

shows that MoI3 crystals grow preferentially along the b-axis, leading to needlelike structures

with high aspect ratios. The energy dispersive spectroscopy (EDS) mapping of the exfoliated

samples demonstrates homogeneous distribution and overlap of Mo and I (Figure 1 (f, g)). The

EDS analytical results also confirm the expected composition of ~1:3 Mo:I (see Supplementary

Information).

F. Kargar et al, Specifics of the Elemental Excitations in “True-1D” MoI3 van der Waals Nanowires (2022)

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[Figure 1: (a) Crystal structure of MoI3 refined in the Pmmn (No. 59) space group; blue and red

spheres represent Mo and I, respectively. The short Mo–Mo interactions of the “dimerized”

MoI3 chains are indicated with yellow bonds. (b) Experimental PXRD 𝜃 − 2𝜃 scan with se-

lected hkl indices. (c) Low magnification HAADF-STEM image and, (d) atomic resolved

HAADF-STEM image of exfoliated MoI3 nanowires. The images confirm that the nanowires

grow along the <010> direction. (e) SEM image of the as-synthesized MoI3. (f, g) Correspond-

ing EDS mapping of the MoI3 crystal confirming the uniform distribution of Mo (red) and I

(green).]

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F.Kargaretal,SpecificsoftheElementalExcitationsin“True-1D”MoI3vanderWaalsNanowires(2022)1|PageSpecificsoftheElementalExcitationsin“True-1D”MoI3vanderWaalsNanowiresFariborzKargar1,*,ZahraBarani1,NicholasR.Sesing2,ThucT.Mai3,TopojitDebnath4,HuairuoZhang5,6,YuhangLiu4,YanbingZhu7,SubhajitGhosh1,AdamJ.B...

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