Giant reversible,facet-dependent,structural changes in a correlated-electron insulator induced by ionic liquid gating

The use of electric fields to alter the conductivity of correlated electron oxides is a powerful tool to probe their fundamental nature as well as for the possibility of developing novel electronic devices. Vanadium dioxide (VO₂) is an archetypical correlated electron system that displays a temperature-controlled insulating to metal phase transition near room temperature. Recently, ionic liquid gating, which allows for very high electric fields, has been shown to induce a metallic state to low temperatures in the insulating phase of epitaxially grown thin films of VO₂. Surprisingly, the entire film becomes electrically conducting. Here, we show, from in situ synchrotron X-ray diffraction and absorption experiments, that the whole film undergoes giant, structural changes on gating in which the lattice expands by up to ∼3% near room temperature, in contrast to the 10 times smaller (∼0.3%) contraction when the system is thermally metallized. Remarkably, these structural changes are fully reversible on reverse gating. Moreover, we find these structural changes and the concomitant metallization are highly dependent on the VO₂ crystal facet, which we relate to the ease of electric-field–induced motion of oxygen ions along chains of edge-sharing VO₆ octahedra that exist along the (rutile) c axis.The use of electric fields to influence the transport properties of various materials by electrostatic injection of charge at an interface is the foundation of much of modern day electronics (1). Using a three-terminal field-effect transistor geometry, the magnitude of the electric fields provided by conventional gate dielectrics is limited by their dielectric properties. Much higher electric fields are possible by replacing the conventional gate material with an ionic liquid. Consequently, much higher electrostatically induced charge densities are possible, leading to the control or creation of novel metallic (2–3) and superconducting phases (4–7). Materials that are insulating by virtue of strong electron–electron correlations, namely Mott–Hubbard and charge-transfer insulators (8), are anticipated to be particularly sensitive to the injection of small numbers of carriers that could result in their metallization (9–11). Often these materials exhibit a thermally driven insulator to metal transition: one of these, VO₂, exhibits such a transition near room temperature (12, 13) and, for this reason, has been extensively studied (14–16). In VO₂ the metal to insulator transition (MIT) is accompanied by a structural phase transition (SPT) in which the monoclinic insulating phase transforms to a rutile metallic phase (17). Recently, both Nakano et al. (18) and Jeong et al. (19) showed that ionic liquid (IL) gating of thin films of VO₂ results in the suppression of the MIT to temperatures below ∼10 K and, moreover, that the entire film becomes metallic even though gating takes place at the top surface of the film in contact with the IL. However, whereas Nakano et al. (18) claimed the metallization phenomenon was a direct result of electrostatic carrier injection, Jeong et al. (19) presented clear evidence that the metallic state was rather induced by the electric-field–induced migration of oxygen from the film into the IL. An important question is whether the IL gating results in a structural phase transition, as supposed by Nakano et al. (18), or whether the initially insulating film remains in the monoclinic phase and the metallization results rather from the formation of oxygen vacancies (19). Here, we show, using in situ X-ray diffraction and absorption, that IL gating induces massive, reversible structural changes in which the VO₂ (001) film expands and contracts along its thickness by up to 3%, but that the film remains in the monoclinic phase. Furthermore, we identify a remarkable dependence of the IL gating phenomenon on the crystal facet of the VO₂ films. Whereas the (001) and (101) facets exhibit similar IL gate-induced metallization, almost no effect is observed for films grown with (100) and (110) facets. Because there are open channels in the VO₂ crystal structure along the rutile c axis, we associate the IL gating phenomenon with the ease of migration of oxygen along these channels under the influence of electric field. Previously, clear evidence for the formation and refilling of oxygen vacancies under ionic liquid gating of VO₂ (001) has been reported (19).The facet-dependent IL-induced metallization and associated structural changes of VO₂ were studied using two types of devices shown in Fig. 1 A and B, respectively. We label these devices as type T and X, respectively. In Fig. 1A a typical device type T with a channel area of 200 × 20 µm² defined by optical lithography is shown (see Methods for details). The channel conductance is measured using the source (S) and drain (D) contacts that are shown in the figure. A drop of the IL (∼100 nl) fully covers the channel and a significant part of the lateral gate electrode (19). Such devices T are suitable for detailed transport studies as a function of temperature and environment. Fig. 1B shows a cell (20) that was specially designed for in situ synchrotron-based X-ray measurements. Device X, used in this cell, is much larger than that of Fig. 1A and indeed is almost as large as the substrate itself (1 × 1 cm²) with S and D gold contacts, defined by shadow masks, along opposite edges of the substrate. The device and the gate electrode, which is formed from a coiled Au wire that surrounds the device, are immersed in ∼2 mL of IL which is introduced through Teflon tubes and which is contained by a 7.5-µm-thick Kapton sheet, sealed with Viton O-rings, that allows for transmission of the incident and diffracted X-ray beams and fluorescent X-rays. The cell is attached to a four-circle X-ray goniometer for the X-ray diffraction studies. Incorporated within the cell is a heater and a Peltier cooler that allows for operation at temperatures ranging from ∼250 K to ∼400 K. Pulsed laser deposition was used to deposit 10-nm-thick VO₂ films with four different crystal facets, (001), (101), (100), and (110) on single-crystalline substrates of TiO₂ with the same respective crystal orientations, and 20-nm-thick VO₂ (001) films on Al₂O₃(101¯0)(see Methods for details).Open in a separate windowFig. 1.Two different types of ionic liquid devices and facet dependence of gating effect. (A) Optical image of a device with a droplet of HMIM-TFSI with channel size of 200 × 20 µm². (B) Optical image of an ionic liquid device for in situ X-ray measurement. The entire film surface (10 × 10-mm² area) is covered with ionic liquid surrounded by a Au wire used as a gate electrode. Source–drain (Top) and gate (Bottom) current versus V_G for small size device (C) and large size device (D) fabricated from VO₂ films grown on TiO₂ substrates of different orientations. Gate voltages were swept at a rate of 3 mV/s (0.3 mV/s for D) and source–drain voltage (V_SD) of 100 mV (300 mV for D) is applied.Fig. 1 C and D compares the gate voltage (V_G) dependence of the source–drain current I_SD and the gate current I_G at 270 K for devices X and T. Initially the devices are in the insulating phase but above a certain threshold gate voltage I_SD increases substantially. When V_G is decreased to zero the devices remain conducting and revert back to their original state only when reverse gated by applying a negative voltage. I_G remains below 5 nA for all V_G for device T. For device X, I_G is much larger but only because of the much larger gated area: the leakage current per unit area of the gated VO₂ is similar for both devices (see SI Appendix for a detailed comparison). In the fully gated state, device T is metallic to low temperatures as shown earlier (19) but device X was only measured to 250 K, due to limitations of the X-ray cell, where it remained metallic. An important result is the dramatic dependence of the IL gating on the VO₂ crystal facet, as is clearly shown in Fig. 1 C and D.X-ray diffraction θ–2θ curves from a VO₂ (001) device X in the insulating phase and the thermally induced metallic phase are shown in Fig. 2A. The unit cell (all peaks are indexed throughout the paper with respect to the rutile unit cell, for simplicity) contracts along the c axis as evidenced by the shift of the VO₂ (002) peak to higher 2θ as VO₂ transforms from the monoclinic to a rutile phase. Fig. 2B shows a sequence of X-ray θ–2θ curves for the device in different gated states as V_G was systematically ramped in steps from 0 to +2.2 V to −2.2 V and back to zero. The X-ray data were collected after V_G was fixed at each step for 30 min. Fig. 2C shows the corresponding values of I_SD for these data. The gate voltage-induced increase in I_SD is ∼3 orders of magnitude. The X-ray data show very large shifts in the VO₂ (002) peak position which, however, are opposite to that seen for the temperature-driven MIT shown in Fig. 2A. The VO₂ (002) peak rather shifts to smaller 2θ values. This corresponds, as shown in Fig. 2D, to an expansion of the c-axis parameter in the fully gated metallic state by a factor which is 10 times larger than the contraction in the c-lattice parameter that is observed for the temperature-driven SPT. We note that the threshold voltages at which the lattice changes are observed appear to be smaller than that at which I_SD changes. We find that no structural changes are observed when the same experiment is carried out on a 10-nm-thick VO₂ film grown with a (110) facet on TiO₂(110). X-ray diffraction θ–2θ curves taken during gating at gate voltages of up to 2.8 V show no shift in the VO₂ (220) peak position nor any other changes in the X-ray diffraction curves. As shown in Fig. 1D there are similarly no changes in I_SD during gating up to V_G = 3 V.Open in a separate windowFig. 2.Structural changes of VO₂/TiO₂(001) by electrolyte gating as a function of gate voltages. (A) XRD patterns of insulating (red) phase at 270 K and metallic phase (black) at 300 K for 10-nm VO₂/TiO₂(001) with 12-keV photon energy. (B) XRD pattern for in situ X-ray measurements and (C) source–drain current (I_SD) versus gate voltage (V_G). Both XRD and I_SD were measured ∼30 min after V_G was applied. (D) c-axis lattice parameter extracted from B versus gate voltage. The error bars are from the nonlinear least-squares fitting algorithm and in many cases are smaller than the symbols.The structural changes that we find on gating VO₂ (001) are similar to those found by growing VO₂ (001) films of comparable thickness at lower oxygen pressures during pulsed laser deposition. Fig. 3A shows X-ray diffraction θ–2θ curves for a series of five samples, each ∼10 nm thick, prepared at oxygen pressures of 9, 7, 5, 3, and 1 mTorr. As the oxygen pressure is reduced the c-lattice parameter systematically increases, as shown in Fig. 3B. The MIT transition is systematically broadened and suppressed to low temperatures as the oxygen pressure is reduced (19). The c-lattice parameter expansion caused by the introduction of oxygen vacancies by modifying the film growth conditions are similar to those that IL gating induces, and the resulting metallization of the VO₂ films is comparable. We presume that the thickness oscillations in the θ–2θ curves from VO₂/TiO₂(001) in Fig. 2B that disappear on gating result from the loss of coherence in the film structure due to gating and the subsequent formation of oxygen vacancies.Open in a separate windowFig. 3.Crystal structure of oxygen-deficient and gated VO₂/TiO₂(001). Dependence of (A) XRD θ–2θ curves, and (B) c-lattice parameter and T_MIT on oxygen pressure during growth. (C) Reciprocal space maps of VO₂(202) peak versus oxygen pressure during film growth and comparison with those for the pristine and gated states of a device formed from a film grown at 9 mtorr. (D) Structure of the monoclinic phase of VO₂ looking along the <001> and <110> axes.The crystal facet of the VO₂ film is determined by epitaxial growth onto the respective facet of the TiO₂ substrate. This also results in clamping of the 10-nm-thick VO₂ films to the corresponding TiO₂ lattices by coherent strain such that their in-plane unit cell parameters are very similar. This is illustrated in Fig. 3C, which shows reciprocal lattice maps centered near TiO₂ (202) in the k = 0 plane for the five films prepared in different oxygen ambients in Fig. 3A. The maps show along the [20l] direction a very sharp, intense TiO₂ (202) peak together with a weaker and broader VO₂ (202) peak and associated Kiessig fringes (21). The VO₂ (202) peak systematically shifts to lower l as the oxygen pressure is reduced. Along the [h02] direction, by contrast, the VO₂ and TiO₂ peaks have similar narrow widths that indicate in-plane clamping of the VO₂ lattice to that of the TiO₂ substrate. Thus, during IL gating it is anticipated that only the out-of-plane VO₂ lattice parameter can be significantly changed. This is confirmed in the reciprocal space map for a sample that was gated at V_G = 3 V for 10 h and the IL removed before the measurement using a laboratory X-ray source.The clamping of the VO₂ lattice to that of the TiO₂ lattice could offer an explanation for the lack of any significant IL gating response for facets of VO₂ for which the c axis lies in plane. It could be that to remove significant amounts of oxygen the lattice needs to expand along the c direction. It is along the rutile c direction that the structure comprises one-dimensional chains of edge-sharing VO₆ octahedra that allow for the expansion and contraction of the VO₂ lattice during IL gating (Fig. 3D).To inspect the local environment of V we performed in situ X-ray absorption spectroscopy (XAS) at the V K edge using VO₂ (001) films grown on Al₂O₃(101¯0)rather than TiO₂ to avoid the otherwise significant fluorescence from Ti in the substrate. The sample was gated and the XAS data were measured at ambient temperature well below the MIT of the pristine film. The X-ray absorption near-edge spectra (XANES) are shown in Fig. 4A for a pristine sample and the same sample after gating (V_G = 3 V, 1 h) to a conducting state. The XANES data remain largely unchanged with two exceptions. Firstly, there is a small shift in the position of the inflection point of the V 1s→3d preedge transition and a small decrease in the intensity of the preedge peak that suggest a reduction in the valence state of the V ions [by ∼0.2 electrons per V (22)]. The decrease in the intensity of this preedge feature is also consistent with a decrease in the degree of distortion of the VO₆ octahedra (22). Secondly, there is a small shift to lower photon energies in the position of the main V K edge and the white line (V1s → V4p transition) at this edge which is also consistent with a reduced V valence on gating (22). A change in V valence was previously observed in X-ray photoelectron spectroscopy measurements performed on electrolyte-gated VO₂ that suggested the formation of oxygen vacancies on gating (19).Open in a separate windowFig. 4.XAS of VO₂/Al₂O₃(101¯0). (A) Vanadium K-edge XANES for a pristine and gated device X. (B and C) χ(R) curves for the data and corresponding fits. The individual contributions to these fits from respective V–O and V–V shells are shown (inverted) in the bottom halves of B and C. The experimental data for the pristine and gated states are shown in blue and red, respectively; the fits to these data are shown in green; the differences between the experimental data and fits are shown in magenta; the contributions from V–O shells are shown in purple; the contributions from V–V shells along the dimer axis are brown and perpendicular to the dimer axis are dark yellow. The brown horizontal lines in B and C are aids to the eye, showing the degree of dimerization, namely, the difference between the intra- and inter-V–V dimer distances. The solid and dashed curves are the moduli and real parts of the Fourier transform of the EXAFS data and the fits, respectively. (D) Table of fitted V–O and V–V bond distances.Much larger changes are observed in the extended X-ray absorption fine structure (EXAFS) (23), χ(k), that is most readily seen in its Fourier transform (FT), namely, χ(R) = FT(k³ χ(k)) (23), that is presented in Fig. 4 B and C. Detailed information on the types of locally ordered neighbor shells and their metrical parameters was obtained by nonlinear least-squares curve fits using calculated amplitudes and phases (24). The k³-weighted data were fit for k varying from 2.6 to 10.3 Å^–1 so that shells are distinguishable only if separated by more than ∼0.15 Å. The spectra were well fit (Fig. 4 B and C) with a limited number of shells relative to the crystal structure (SI Appendix). The distances found for the pristine samples are consistent with the monoclinic phase of VO₂ below its MIT in which the shorter V–V distances that correspond to those in the (rutile) c direction that is normal to the film plane are split because of the dimerization of the V–V atoms along this direction. In the pristine sample these V–V distances of 2.61 and 3.03 Å differ by 0.42 Å, whereas in the gated sample these distances become 2.94 and 3.16 Å and differ by only 0.22 Å. Note that the almost complete loss of the peak in the χ(R) spectra near R−ϕ = 2.0 Å on gating is not because of a radical change in the V–V chain ordering and departure from the V–V dimerized monoclinic structure, but is because the decreased separation between the short and long dimer V–V pairs causes their individual EXAFS waves to destructively interfere, reducing their combined amplitude in the FT. Thus, a crucial result is that the V–V dimerization, although reduced, is nevertheless retained in the gated state. Moreover, the average V–V distance in the c direction normal to the film planes increases by a much larger amount than is found by diffraction that could indicate rotation of the VO₆ octahedra. On the other hand, the V–V distance within the ab plane (∼3.5 Å) changes little on gating (Fig. 4D), suggesting that the structure in the plane is largely unaffected, consistent with the X-ray diffraction data in Fig. 3C.Our in situ X-ray diffraction (XRD) and XAS measurements clearly indicate a giant expansion of the VO₂ unit cell that is clearly inconsistent with the formation of the rutile phase that the thermally induced metallic phase exhibits. Moreover, we find these reversible structural changes only in films in which channels formed from chains of edge-sharing VO₆ octahedra do not lie in the plane of the films, strongly suggesting that these channels are the paths along which the gate-induced oxygen migration takes place. These gate-induced changes in structure and conductivity are likely to be common to many ionic liquid gated systems, opening the way to a potential future of “liquid electronics.”     

Effect of large volume replacement with balanced electrolyte solutions on extravascular lung water in surgical patients with sepsis syndrome

We investigated the effect of large volume replacement with balanced electrolyte solutions on extravascular lung water (EVLW) in 16 adult surgical patients with sepsis syndrome. Patients entered the study within the 24 h period following surgical interventions for acute necrotizing pancreatitis, intra-abdominal abscesses, and/or peritonitis. Sequential measurements (n=108) were made at intervals of 6–12 h over a 48 h period. There were no significant differences between initial and final values of thermal-dye EVLW (5.0±1.1 vs. 5.7±1.1 ml/kg), plasma colloid osmotic pressure (COP, 13.3±2.5 vs. 13.2±2.9 mmHg), pulmonary artery wedge pressure (PAWP, 9.2±3.0 vs. 10.8±3.0 mmHg), and COP-PAWP gradient (4.0±3.5 vs. 2.4±3.9 mmHg). All results expressed as (mean±SD). The EVLW did not correlate with plasma COP, PAWP, or COP-PAWP gradient. We conclude that large volume replacement with balanced electrolyte solutions with the secondary decrease in plasma COP and COP-PAWP gradient do not necessarily contribute to a substantial increase in EVLW. This study fails to show any causal relationship between decrease in plasma COP or COP-PAWP gradient and oedema formation in the lung.Supported by the Austrian Govenment, Department of Health     

结肠镜术前肠道准备三种方案清洁效果的对比研究

目的 比较临床上常用的3种肠道准备方案的肠道清洁效果,寻求最快速有效的肠道准备方案。方法 对2017年5月至6月在江苏省人民医院消化科病房拟行肠镜检查的患者77例行不同方案的肠道准备,并按不同方案分为3组。第1组22例口服聚乙二醇电解质散2 L+乳果糖口服液6包单次服用;第2组31例口服聚乙二醇电解质散2 L+乳果糖口服液6包分次服用;第3组24例口服聚乙二醇电解质散3 L。观察患者服药后肠道准备的效果和不良反应情况。结果 肉眼观察肠道清洁有效的例数,第2组明显多于其他两组（P＜0.05）;肠镜下渥太华量表评分显示,第1组和第2组对比差异无统计学意义（P＞0.05）,第2组和第3组对比,差异有统计学意义（P＜0.05）,说明渥太华评分标准判断下,第2组优于第3组;服药不良反应率比较,第2组明显优于其他两组（P＜0.05）。结论 采用聚乙二醇电解质散2 L+乳果糖口服液6包分次服用方案进行肠道准备,具有较好的肠道清洁效果,不良反应少,患者更易接受。     

特利加压素导致低钠血症性脑病的病例分析并文献复习

目的： 提醒临床医师及药师在临床工作中应警惕特利加压素可能导致的低钠血症性脑病。方法： 报道1例使用特利加压素出现低钠血症性脑病最终导致患者死亡的案例,并复习国内外相关文献进行分析讨论。结果： 检索到3篇使用特利加压素至低钠血症性脑病的个案报道,但均未引起影像学改变且停药后神经系统症状得以恢复;在回顾性研究中,发现使用特利加压素发生低钠血症的危险因素可能有：年龄小、基线高血钠水平、长疗程（>5 d）以及较好的肝功能（低MELD或Child评分）。结论： 特利加压素引起的低钠血症可能导致不可逆的神经系统并发症,使用该药期间应严密监测血钠,尤其是有高危因素的患者。     

The osmotic behaviour of toad skin epithelium (Bufo viridis)

The effect of saline adaptation on the intracellular Na, K, Cl, P concentrations and dry weight content of the toad skin epithelium (Bufo viridis) was studied using the technique of electron microprobe analysis. The measurements were performed on isolated abdominal skins either directly after dissection or after additional incubation in Ussing-type chambers.Adaptations of the toads to increasing NaCl concentrations for 7 days resulted in increased blood plasma osmolarity and a parallel increase in the cellular electrolyte, P and dry weight concentrations of the epithelium, the K increase representing the most significant fraction of the intracellular osmolarity increase. No evidence was obtained to show that the nucleus and cytoplasm reacted differently from each other and all living epithelial cell types basically showed the same response.Incubation of the isolated skins under control conditions showed a drastic inhibition of the transepithelial Na transport after adaptation to high salinities. In spite of the large variations in the transport rate almost identical intracellular electrolyte concentrations were observed. In tap water adapted toads the average cellular concentrations were 8.8 mmole/kg wet weight for Na, 109.6 for K, 41.5 for Cl, and 135.3 for P, respectively. Incubation of the skin with Ringer's solution of different osmolarities demonstrated that the epithelial cells are in osmotic equilibrium with the inner bathing solution. The results are consistent with the view that the osmotic adaptation is mainly accomplished by the movement of water.This work was supported by grants from the Deutsche Forschungsgemeinschaft and the Stiftung Volkswagenwerk     

Investigation of the Role of Kidney Kallikrein on Bumetanide Induced Diuresis in Rats

Abstract: Administered to anaesthetized, laparatomized rats bumetanide (10 mg/kg intravenously) evoked a rapid and large diuretic response. This was associated with three to five fold enhancements of urine kallikrein (kininogenase) excretion. In addition bumetanide slightly decreased (approximately 10 mmHg) the blood pressure and temporarily increased renal blood flow (<10%). The capacity of bumetanide to increase urine kallikrein excretion was inhibited either by pretreatment with chlorazanil (3 mg/kg intravenously) or by the infusion of aprotinin (5000 KIE/kg/min.). Kallikrein inhibition did not significantly modify bumetanide induced diuresis, natriuresis, kaliuresis, lowering of blood pressure or renal vasodilating effect.     

Effect of Repeated Doses of Hydroflumethiazide on Renal Excretion of Electrolytes and Uric Acid in Healthy Subjects

Abstract: Urinary excretion of electrolytes and uric acid was investigated in six healthy subjects during repeated oral administration of 100 mg hydroflumethiazide (HFT) daily for seven days, and related to urinary thiazide excretion. Mean 24 hr-urinary excretion of sodium and chloride increased 100% (P<0.02) after the first HFT-dose, whereas 24 hr-excretion values were at control level after the fourth and seventh doses. Mean 24 hr-urinary excretion of potassium was increased by 31% after the first HFT-dose (P<0.05) and by 47% after the fourth dose (P <0.05). After HFT was discontinued, mean urinary excretion rates of sodium and chloride dropped to 30% and that of potassium to 70% of control. In the state of fluid deficiency and elevated aldosterone concentration, there was a significant positive correlation between log excretion rate of HFT and excretion rate of sodium (r=0.68, P<0.002) calculated from excretion data 0–6, 6–12, and 12–14 hrs after the seventh dose. After the first dose of HFT, sodium excretion was also significantly correlated to log excretion rate of HFT (r=0.86, P<0.001) but was probably influenced by other factors as well. Mean serum concentration of uric acid increased significantly, but mean 24 hr-urinary excretion of uric acid was constant during HFT-treatment.     

结直肠术后患者结肠镜检查前的肠道准备方法探讨

目的改进结直肠术后患者肠道准备方法,提高结肠镜检查质量。方法选择拟行结肠镜检查的结直肠术后患者100例,分为两组,各50例。A组患者(同术前准备方法)结肠镜检查前5 h口服复方聚乙二醇电解质散(SF-PEG)328.8 g(3 000 ml);B组患者结肠镜检查前1日3餐后2 h分别服用SF-PEG27.4 g(250 ml),检查前5 h口服50%硫酸镁(MgSO4)100 ml,再喝温开水1 000 ml,至排泄液似清水样。服药后5 h行结肠镜检查。应用Boston肠道准备量表(BBPS)评分,对肠腔内气泡进行评分,比较两组患者肠道准备有效性、耐受性及安全性。结果 B组的结肠清洁程度BBPS总体评分(8.50±0.35)分,高于A组(7.35±1.25)分;B组进镜时间(3.85±1.20)min和退镜时间(6.25±0.60)min,少于A组进镜时间(5.35±1.75)min和退镜时间(8.20±0.85)min,差异均有统计学意义(P0.05)。B组患者肠道准备接受率、再次肠道准备接受率和总体不良反应评分分别为96.0%、94.0%和(1.35±0.05)分;A组患者分别为86.0%、72.0%和(1.75±0.30)分;差异均有统计学意义(P0.05),B组优于A组。结论结直肠术后患者肠镜检查前的肠道准备,采用间断冲击口服小剂量复方SF-PEG联合MgSO4,效果优于常规剂量。