Local nano-mechanical properties in twisted double bi-layer graphene

Van der Waals heterostructures are tremendously versatile designer materials whose functionality can be engineered to an extend that goes far beyond the properties of the individual materials the heterostructure consists of [1]. In particular, by twisting two graphene layers, it is possible to induce an atomic reconstruction in the two-dimensional stack, which leads to a dramatic modification of the lattice symmetry [2]. This has important repercussions on its mechanical and electro-mechanical properties [3,4]. Here we investigate the local mechanical properties of double bi-layer graphene twisted by an angle ~1.1°. To this end, we employ three force microscope techniques, Piezoresponse Force Microscopy, Ultrasonic Force Microscopy and Electric Heterodyne Force Microscopy, respectively. We demonstrate that these methods are reliable and effective to visualize the Moiré pattern, to evidence the presence of strain solitons [5], and – for the first time – to extract the local Youngs modulus in such systems. Our results bring on a comprehensive study of such complex structures and unlock critical understanding of these materials. References [1] Geim, A., Grigorieva, I., Nature, 499 (2013) 419–425. [2] Dai, S., Xiang, Y., Srolovitz, D. J., Nano Lett., 16, 9 (2016) 5923–5927. [3] De Sanctis, A., Mehew, J. D., et al., Nano Lett., 18, 12 (2018) 7919–7926. [4] Li, Y., Wang, Xet al., Adv. Mater., 33 (2021) 2105879. [5] Alden, J. S., Tsen, A. W., et al., PNAS, 110 (2013) 11256–11260

Quantifying the local mechanical properties of twisted double bilayer graphene

Nanomechanical measurements of minimally twisted van der Waals materials remained elusive despite their fundamental importance for device realisation. Here, we use Ultrasonic Force Microscopy (UFM) to locally quantify the variation of out-of-plane Young's modulus in minimally twisted double bilayer graphene (TDBG). We reveal a softening of the Young's modulus by 7% and 17% along single and double domain walls, respectively. Our experimental results are confirmed by force-field relaxation models. This study highlights the strong tunability of nanomechanical properties in engineered twisted materials, and paves the way for future applications of designer 2D nanomechanical systems

The Enhancement of Mg Corrosion Resistance by Alloying Mn and Laser-Melting

Mg has been considered a promising biomaterial for bone implants. However, the poor corrosion resistance has become its main undesirable property. In this study, both alloying Mn and laser-melting were applied to enhance the Mg corrosion resistance. The corrosion resistance, mechanical properties, and microstructure of rapid laser-melted Mg-xMn (x = 0–3 wt %) alloys were investigated. The alloys were composed of dendrite grains, and the grains size decreased with increasing Mn. Moreover, Mn could dissolve and induce the crystal lattice distortion of the Mg matrix during the solidification process. Mn ranging from 0–2 wt % dissolved completely due to rapid laser solidification. As Mn contents further increased up to 3 wt %, a small amount of Mn was left undissolved. The compressive strength of Mg-Mn alloys increased first (up to 2 wt %) and then decreased with increasing Mn, while the hardness increased continuously. The refinement of grains and the increase in corrosion potential both made contributions to the enhancement of Mg corrosion resistance

PFM and s-SNOM imaging of minimally-twisted double bilayer graphene: moirés, domain walls and solitons

posterInternational audienceTwisting graphene-based heterostructures results in local atomic reconstructions which have large consequences in terms of local and global electronic and optical properties [1]. At small twist angle, the moiré superstructure relaxes by forming domains with homogeneous atomic ordering, separated by domain walls (DWs), which play a major role in the observed local and global properties. In addition, other DWs can be observed, called solitons, analogous to dislocations in the crystal [2-4]. Topologically protected edge channels in the valley quantum Hall regime have been observed at the edge of such DWs in bilayer graphene [2]. In this study, we have characterized the DWs superlattices in twisted double bilayer graphene at very small twist angle by piezoresponse force microscopy (PFM) and scattering-type scanning near-field optical microscopy (s- SNOM) in the mid-infrared range (9 µm - 10.6 µm). These two techniques provide complementary information. PFM imaging (Fig. 1a) mainly reveals a contrast related to DWs, via flexoelectric effects, bond tensions and atomic orbital alterations. SNOM imaging (Fig. 1b) provides information on local optical conductivity changes within the domains (corresponding to different local stacking) and to the generation of plasmons launched by the tip and reflected at the walls. We also observe solitons that can be manipulated (moved, wiped out) via the action of the tip

Evaluating the Quality of Cardiopulmonary Resuscitation in the Emergency Department by Real-Time Video Recording System.

To compare cardiopulmonary resuscitation (CPR) quality between manual CPR and miniaturized chest compressor (MCC) CPR. To improve CPR quality through evaluating the quality of our clinical work of resuscitation by real-time video recording system.The study was a retrospective observational study of adult patients who experienced CPR at the emergency department of Shanghai Tenth People's Hospital from March 2013 to August 2014. All the performance of CPR were checked back by the record of "digital real-time video recording system". Average chest compression rate, actual chest compression rate, the percentage of hands-off period, time lag from patient arrival to chest compression, time lag from patient arrival to manual ventilation, time lag from patient arrival to first IV establish were compared. Causes of chest compression hands-off time were also studied.112 cases of resuscitation attempts were obtained. Average chest compression rate was over 100 compression per minute (cpm) in the majority of cases. However, indicators such as percentage of hands-off periods, time lag from patient arrival to the first manual ventilation and time lag from patient arrival to the first IV establish seemed to be worse in the manual CPR group compared to MCC CPR group. The saving of operators change time seemed to counteract the time spent on MCC equipment. Indicators such as percentage of hands-off periods, time lag between patient arrival to the first chest compression, time lag between patient arrival to the first manual ventilation and time lag from patient arrival to the first IV establish may influence the survival.Our CPR quality remained to be improved. MCC may have a potentially positive role in CPR

PFM and s-SNOM imaging of minimally-twisted double bilayer graphene: moirés, domain walls and solitons

posterInternational audienceTwisting graphene-based heterostructures results in local atomic reconstructions which have large consequences in terms of local and global electronic and optical properties [1]. At small twist angle, the moiré superstructure relaxes by forming domains with homogeneous atomic ordering, separated by domain walls (DWs), which play a major role in the observed local and global properties. In addition, other DWs can be observed, called solitons, analogous to dislocations in the crystal [2-4]. Topologically protected edge channels in the valley quantum Hall regime have been observed at the edge of such DWs in bilayer graphene [2]. In this study, we have characterized the DWs superlattices in twisted double bilayer graphene at very small twist angle by piezoresponse force microscopy (PFM) and scattering-type scanning near-field optical microscopy (s- SNOM) in the mid-infrared range (9 µm - 10.6 µm). These two techniques provide complementary information. PFM imaging (Fig. 1a) mainly reveals a contrast related to DWs, via flexoelectric effects, bond tensions and atomic orbital alterations. SNOM imaging (Fig. 1b) provides information on local optical conductivity changes within the domains (corresponding to different local stacking) and to the generation of plasmons launched by the tip and reflected at the walls. We also observe solitons that can be manipulated (moved, wiped out) via the action of the tip

Non-identical moiré twins in bilayer graphene

The superlattice obtained by aligning amonolayer graphene and boron nitride (BN) inherits from the hexagonal lattice a sixty degrees periodicity with the layer alignment. It implies that, in principle, the properties of the heterostructure must be identical for 0° and 60° of layer alignment. Here, we demonstrate, using dynamically rotatable van der Waals heterostructures, that the moiré superlattice formed in a bilayer graphene/BN has different electronic properties at 0° and 60° of alignment. Although the existence of these non-identical moiré twins is explained by different relaxation of the atomic structures for each alignment, the origin of the observed valley Hall effect remains to be explained. A simple Berry curvature argument is not sufficient to explain the 120° periodicity of this observation. Our results highlight the complexity of the interplay between mechanical and electronic properties in moiré structures and the importance of taking into account atomic structure relaxation to understand their electronic properties

CPR Parameters During Cardiac Arrest Episodes.

<p>Note: The patients were divided into survival and non-survival group depending on the survival to be hospitalized. cpm: compression per minute. AVCR = (N: total chest compression counts) / (T1: total chest compression time). ACCR = N / (T1—T2) (T2: hands-off time). Hands-off time was defined as the time that was longer than 1s between two consecutive compressions. Percentage of hands-off time = T2 / T1. Data was presented as median (interquartile ranges) (non-parametric).</p><p>CPR Parameters During Cardiac Arrest Episodes.</p

Demographic and Descriptive Clinical Data.

<p>Note: The patients were divided into manual CPR and MCC CPR group depending on whether the patients were applied with MCC after arriving at the hospital, the chest compression out-of-hospital was not counted. Data are presented as counts (percentage) (95% CI) of population except age [mean (95% CI)] (parametric).</p><p>Demographic and Descriptive Clinical Data.</p