Effects of electron‐beam‐generated anion point defects on the long range order of charge density waves in 1
T
‐
T
a
S
e2, 1
T
‐
T
a
S
2
Authors: Kinyanjui Michael , Boerner Pia , Lehnert Tibor , Koster Janis , Kaiser Ute 
Published: December 2016

Charge density waves (CDW) are periodic modulations of charge density in low‐dimensional metals observed as a function of temperature, doping and pressure. Due to electron‐phonon coupling CDWs, are also accompanied by a periodic lattice distortion (PLD).
1
Quasi two‐dimensional (2D) transition metal dichalcogenides metals like 1T/2H‐TaSe
2
, 1T/2H‐TaS
2
, 2H‐NbSe
2
are some of the materials that exhibit strong CDW distortions. While CDW can be directly probed using Scanning Tunnelling Microscopy (STM), diffraction and imaging techniques such as High Resolution Transmission electron Microscopy (HRTEM) and selected area electron diffraction (SAED) are sensitive to the structural distortions (PLD) accompanying CDW. In an electric field, and in the absence of crystalline impurities and defects, sliding incommensurate CDW show transport properties similar to a superconducting state.
1,2
It is therefore important to understand the effects of commensuration, defects, and impurities on the static and dynamic properties of the CDW state.
2,3
On a transmission electron microscope (TEM), investigating the effects of defects on the CDW/PLD state can be done in‐situ by generating point defects through electron beam irradiation and at the same time monitoring the structure of the CDW/PLD through electron diffraction and atomic resolved HRTEM imaging.

Here we report on the interaction of commensurate CDW/PLD with point defects generated by the electron beam in 1T‐TaSe
2
and 1T‐TaS
2
. Due to the CDW/PLD, bulk 1T‐TaS
2
and 1T‐TaSe
2
are characterized by a commensurate √13 a
0
×√13 a
0
(a
0
= 3.447) superlattice at 180 K and 300 K respectively.
1
Figure 1(a) displays a HRTEM image of 1T‐TaSe
2
obtained at 100 K. The HRTEM image was obtained at region consisting of upto 10‐15 layers of 1T‐TaSe
2
and shows Ta atomic columns. The HRTEM image is also characterized by a bright, dark contrast modulation due to the PLD/CDW. A Fast Fourier Transform (FFT) of the HRTEM image shown in fig 1(b) has spots from the main structure (solid red circles) surrounded by six satellite spots (dotted circle) from the PLD/CDW. HRTEM image analysis involving masking of the satellite spots from the CDW/PLD with a circular mask, followed by inverse FFT helps to visualize the structure of the lattice arising from the commensurate PLD/CDW. The resulting image, shown in fig. 1(c), then displays an intensity image showing a √13 a
0
×√13 a
0
lattice of the CDW/PLD maxima.

To investigate the effects of electron‐beam generated defects on the CDW/PLD, succesive HRTEM images were obtained from the same sample region over time and at a constant electron dose. Images showing the evolution of PLD/CDW maxima with electron beam irradiation were then obtained from respective HRTEM images using the image analysis procedure elucidated above. Figures 1(d)‐(f) show successive HRTEM images obtained over time. The HRTEM images were obtained from a region of upto 15 layers of 1T‐TaSe
2
and only show the Ta atomic columns. The PLD/CDW lattices shown in fig.1(g), 1(h), 1(i) correspond to the HRTEM images in figs. 1(d), 1(e), and 1(f) respectively. A radial distribution function (RDF) showing the nearest‐neighbor and next‐nearest‐neighbor periodicities between the CDW maxima can also be calculated. The RDF in figs.1(j), 1(k), 1(l) have been calculated from the CDW/PLD lattices in figs. (g), (h), (i) respectively. The peak with the asterix shows the main periodicity due to the CDW/PLD wave‐vector |q

i

=1…6
| = √13 a
0
. Loss of long‐range order with increased exposure to the electron beam is characterized by broadening of RDF peaks (see regions marked with dotted rectangle in fig. 1(k) and fig.1 (l)). Based on the characterization of 1T‐TaSe
2
, and 1T‐TaS
2
thin/single layers we show that this loss of long range order is due to interaction of CDW/PLD with S and Se anionic point defects generated by the electron beam.