Positive non-linear capacitance: the origin of the steep subthreshold-slope in ferroelectric FETs.

We show that the non-linear positive capacitance (PC) of ferroelectrics (FE) can explain the steep subthreshold-slope (SS) observed in FE based MOSFETs and often attributed to the existence of a negative capacitance in FE capacitors. Physically attainable and unattainable regions of the S-shape curve used in the negative capacitance theory are investigated by self-consistently solving Landau-Khalatnikov and Maxwell equations and by experimental validation. Finally, the conditions for attaining a steep SS in FE based MOSFETs assuming only positive capacitances are discussed.

Chemical vapor deposition of monolayer-thin WS2 crystals from the WF6 and H2S precursors at low deposition temperature.

Monolayer-thin WS2 with (0002) texture grows by chemical vapor deposition (CVD) from gas-phase precursors WF6 and H2S at a deposition temperature of 450 °C on 300 mm Si wafers covered with an amorphous Al2O3 starting surface. We investigate the growth and nucleation mechanism during the CVD process by analyzing the morphology of the WS2 crystals. The CVD process consists of two distinct growth regimes. During (i) the initial growth regime, a fast and self-limiting reaction of the CVD precursors with the Al2O3 starting surface forms predominantly monolayer-thin WS2 crystals and AlF3 crystals that completely cover the starting surface. During (ii) the steady-state growth regime, a much slower, anisotropic reaction on the bottom, first WS2 layer proceeds with the next WS2 layer growing preferentially in the lateral dimensions. We propose that the precursor adsorption reaction rate strongly diminishes when the precursors have no more access to the Al2O3 surface as soon as the WS2 layer completely covers the Al2O3 surface and that the WS2 crystal basal planes and AlF3 crystals have a low reactivity for WF6 adsorption at 450 °C. Nonetheless, a second layer of WS2 starts to form before the first WS2 layer completely covers the starting surface, albeit the surface coverage of the second layer is low (<20%, after 25 min of CVD reaction). During the steady-state growth regime, predominantly the WS2 crystals in the second monolayer continue to grow in lateral dimensions up to ∼40 nm. These crystals reach larger lateral dimensions compared to the crystals in the bottom, first layer due to low reactivity for WF6 adsorption on the WS2 basal plane compared to Al2O3. Presumably, they grow laterally by precursor species that adsorb on and diffuse across the WS2 surface, before being incorporated at the more reactive edges of the WS2 crystals in the second layer. Such a process proceeds slowly with only up to 40% surface coverage of the second WS2 layer after 150 min of CVD reaction. The CVD reaction is mediated by the starting surface: WF6 precursor preferentially adsorbs on Al2O3, whereas adsorption is not observed on SiO2. Nevertheless, WS2 grows on SiO2 in close proximity to Al2O3 in 90 nm pitch Al2O3/SiO2 line patterns. Hence, functionalization of the starting surface (e.g., SiO2 with Al2O3) can provide opportunities to grow monolayer-thin WS2 crystals at predetermined locations by selective, lateral growth with tunable crystal size, even at low deposition temperatures.

Low temperature deposition of 2D WS2 layers from WF6 and H2S precursors: impact of reducing agents.

We demonstrate the impact of reducing agents for Chemical Vapor Deposition (CVD) and Atomic Layer Deposition (ALD) of WS2 from WF6 and H2S precursors. Nanocrystalline WS2 layers with a two-dimensional structure can be obtained at low deposition temperatures (300-450 °C) without using a template or anneal.

Switching mechanism in two-terminal vanadium dioxide devices.

Two-terminal thin film VO2 devices show an abrupt decrease of resistance when the current or voltage applied exceeds a threshold value. This phenomenon is often described as a field-induced metal-insulator transition. We fabricate nano-scale devices with different electrode separations down to 100 nm and study how the dc switching voltage and current depend on device size and temperature. Our observations are consistent with a Joule heating mechanism governing the switching. Pulsed measurements show a switching time to the high resistance state of the order of one hundred nanoseconds, consistent with heat dissipation time. In spite of the Joule heating mechanism which is expected to induce device degradation, devices can be switched for more than 10(10) cycles making VO2 a promising material for nanoelectronic applications.

Physical forces exerted by microbubbles on a surface in a traveling wave field.

The effect of a wave with a varying traveling component on the bubble activity as well as the physical force generated by microbubbles on a surface has been studied. The acoustic emission from a collection of bubbles is measured in a 928 kHz sound field. Particle removal tests on a surface, which actually measures the applied physical force by the bubbles on that surface, indicate a very strong dependence on the angle of incidence. In other words, when the traveling wave component is maximized, the average physical force applied by microbubbles reaches a maximum. Almost complete particle removal for 78 nm silica particles was obtained for a traveling wave, while particle removal efficiency was reduced to only a few percent when a standing wave was applied. This increase in particle removal for a traveling wave is probably caused by a decrease in bubble trapping at nodes and antinodes in a standing wave field.

Enhancement of cavitation activity and particle removal with pulsed high frequency ultrasound and supersaturation.

Megasonic cleaning as applied in leading edge semiconductor device manufacturing strongly relies on the phenomenon of acoustic cavitation. As the occurrence of acoustic cavitation is incorporating a multitude of interdependent effects, the amount of cavitation activity in the cleaning liquid strongly depends on the sonication conditions. It is shown that cavitation activity as measured by means of ultraharmonic cavitation noise can be significantly enhanced when pulsed sonication is applied to a gas supersaturated liquid under traveling wave conditions. It is demonstrated that this enhancement coincides with a dramatic increase in particle removal and is therefore of great interest for megasonic cleaning applications. It is demonstrated that the optimal pulse parameters are determined by the dissolution time of the active bubbles, whereas the amount of cavitation activity depends on the ratio between pulse-off and pulse-on time as well as the applied acoustic power. The optimal pulse-off time is independent of the corresponding pulse-on time but increases significantly with increasing gas concentration. We show that on the other hand, supersaturation is needed to enable acoustic cavitation at aforementioned conditions, but has to be kept below values, for which active bubbles cannot dissolve anymore and are therefore lost during subsequent pulses. For the applicable range of gas contents between 100% and 130% saturation, the optimal pulse-off time reaches values between 150 and 340 ms, respectively. Full particle removal of 78 nm-diameter silica particles at a power density of 0.67 W/cm(2) is obtained for the optimal pulse-off times. The optimal pulse-off time values are derived from the dissolution time of bubbles with a radius of 3.3 µm and verified experimentally. The bubble radius used in the calculations corresponds to the linear resonance size in a 928 kHz sound field, which demonstrates that the recycling of active bubbles is the main enhancement mechanism. The optimal choice of the pulsing conditions however is constrained by the trade-off between the effective sonication time and the desire to have a sufficient amount of active bubbles at lower powers, which might be necessary if very delicate structures have to be cleaned.

Towards an understanding and control of cavitation activity in 1 MHz ultrasound fields.

Various industrial processes such as sonochemical processing and ultrasonic cleaning strongly rely on the phenomenon of acoustic cavitation. As the occurrence of acoustic cavitation is incorporating a multitude of interdependent effects, the amount of cavitation activity in a vessel is strongly depending on the ultrasonic process conditions. It is therefore crucial to quantify cavitation activity as a function of the process parameters. At 1 MHz, the active cavitation bubbles are so small that it is becoming difficult to observe them in a direct way. Hence, another metrology based on secondary effects of acoustic cavitation is more suitable to study cavitation activity. In this paper we present a detailed analysis of acoustic cavitation phenomena at 1 MHz ultrasound by means of time-resolved measurements of sonoluminescence, cavitation noise, and synchronized high-speed stroboscopic Schlieren imaging. It is shown that a correlation exists between sonoluminescence, and the ultraharmonic and broadband signals extracted from the cavitation noise spectra. The signals can be utilized to characterize different regimes of cavitation activity at different acoustic power densities. When cavitation activity sets on, the aforementioned signals correlate to fluctuations in the Schlieren contrast as well as the number of nucleated bubbles extracted from the Schlieren Images. This additionally proves that signals extracted from cavitation noise spectra truly represent a measure for cavitation activity. The cyclic behavior of cavitation activity is investigated and related to the evolution of the bubble populations in the ultrasonic tank. It is shown that cavitation activity is strongly linked to the occurrence of fast-moving bubbles. The origin of this "bubble streamers" is investigated and their role in the initialization and propagation of cavitation activity throughout the sonicated liquid is discussed. Finally, it is shown that bubble activity can be stabilized and enhanced by the use of pulsed ultrasound by conserving and recycling active bubbles between subsequent pulsing cycles.

Time-resolved monitoring of cavitation activity in megasonic cleaning systems.

The occurrence of acoustic cavitation in the cleaning liquid is a crucial precondition for the performance of megasonic cleaning systems. Hence, a fundamental understanding of the impact of different parameters of the megasonic process on cavitation activity is necessary. A setup capable of synchronously measuring sonoluminescence and acoustic emission originating from acoustically active bubbles is presented. The system also includes a high-speed-stroboscopic Schlieren imaging system to directly visualize the influence of cavitation activity on the Schlieren contrast and resolvable bubbles. This allows a thorough characterization of the mutual interaction of cavitation bubbles with the sound field and with each other. Results obtained during continuous sonication of argon-saturated water at various nominal power densities indicate that acoustic cavitation occurs in a cyclic manner, during which periods of stable and inertial cavitation activity alternate. The occurrence of higher and ultraharmonics in the acoustic emission spectra is characteristic for the stable cavitation state. The inertial cavitation state is characterized by a strong attenuation of the sound field, the explosive growth of bubbles and the occurrence of broadband components in the acoustic spectra. Both states can only be sustained at sufficiently high intensities of the sound field. At lower intensities, their occurrences are limited to short, random bursts. Cleaning activity can be linked to the cavitation activity through the measurement of particle removal on standard 200 mm silicon wafers. It is found that the particle removal efficiency is reduced, when a continuous state of cavitation activity ceases to exist.

A strategy towards professionalism in the dissecting room

The teaching and cultivation of professionalism is an integral part of medical education as professionalism is central to maintaining the public's trust in the medical profession. Traditionally professional values would have been acquired through an informal process of socialisation and observation of role models. Recently, however, medical educators have accepted the responsibility to explicitly teach and effectively evaluate professionalism. A comprehensive working definition of the term professionalism and a universally agreed list of the constituent elements of professionalism are currently debated. The School of Medicine and Dentistry of The Queen's University of Belfast uses an approach of self-directed learning for teaching anatomy, and students are given the opportunity to learn anatomy from human dissection. Self-directed learning teams have been found to be underutilised as educational strategies and presented an opportunity to utilise the first year dissection room teaching environment to nurture the development of the attributes of professionalism. An educational strategy based on role-playing was developed to engage all students around the dissection table. Students received comprehensive background reviews on professionalism, its attributes and the identification of such attributes in the context of the dissection room. Roles, with specific duties attached, were allocated to each team member. Circulating academic staff members directly observed student participation and gave formative feedback. Students were given the opportunity to reflect on their ability to identify the attributes and reflect on their own and their peer's ability to develop and practise these attributes. This strategy indicated that small group learning teams in the dissection room utilise widely accepted principles of adult learning and offer an opportunity to create learning activities that will instil in students the knowledge, values, attitudes and behaviours that characterise medical professionalism. Anatomy faculty have a responsibility to nurture and exemplify professionalism and play a significant role in the early promotion and inculcation of professionalism. It remains imperative not only to assess this strategy but also to create opportunities for critical reflection and evaluation within the strategy (AU)

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Shock absorption potential of different mouth guard materials.

STATEMENT OF PROBLEM: Clinical stresses and situations in the mouths of patients are substantially different; thus, it is not possible to make an exact in vivo comparison of the efficiency of various mouth guards. PURPOSE: This study developed a device and a method to test and quantify the potential of mouth guards to absorb shock and evaluated and compared 5 designs for mouth guards. MATERIAL AND METHODS: Strain gauges and accelerometer sensors were mounted in various positions on the maxilla and inside an artificial skull. Sensors quantified the response of several areas of the skull to inputs of force on the maxillary teeth protected by the mouth guard being investigated. Input of force was applied to the maxillary teeth with a modal hammer equipped with a load cell. Five mouth guards of each of 5 designs were manufactured and placed in position before force was applied to the maxillary teeth. All inputs were measured and analyzed and, from those results, a method was developed to quantify the relative potential of the mouth guards to absorb shock. This method was then used to quantify the shock absorption potential of the 5 types of mouth guard designs. RESULTS: All 5 types of mouth guards provided some measure of protection and were better than no protection. However, the level of protection provided by the 5 mouth guard designs differed.

Acute myocardial infarction due to coronary vasospasm secondary to industrial nitroglycerin withdrawal. A case report.

A case of acute transmural anterior myocardial infarction in a 45-year-old Black employee of an explosives factory during a period of withdrawal from industrial nitroglycerin is documented. Angiography revealed that the patient had normal coronary arteries. Coronary vasospasm could not be induced by the ergometrine (ergonovine) maleate provocation test. It is postulated that the infarction was directly attributable to coronary vasospasm provoked by the 'industrial nitroglycerin withdrawal syndrome', since there was no evidence of any other non-atheromatous aetiological factor. The authors believe this to be the first such case in a Black subject reported in the literature.

Acute coronary vasospasm secondary to industrial nitroglycerin withdrawal. A case presentation and review.

A Black employee exposed to industrial nitroglycerin (NG) in an explosives factory presented with severe precordial pain. The clinical presentation was that of significant transient anteroseptal and anterolateral transmural myocardial ischaemia which responded promptly to sublingual isosorbide dinitrate. Despite being removed from exposure to industrial NG and receiving therapy with long-acting oral nitrates and calcium antagonists, the patient continued to experience repeated attacks of severe retrosternal pain, although transient myocardial ischaemia was not demonstrated electrocardiographically during these episodes. Cardiac catheterization revealed a normal myocardial haemodynamic system and selective coronary arteriography delineated coronary arteries free from any obstructive lesions. An ergonovine (ergometrine) maleate provocative test failed to elicit coronary artery spasm, although this was undertaken while the patient was on nitrate and calcium-blocker therapy. Clinical records of previous significant constrictive pericarditis (probably due to tuberculosis) with resultant abnormalities on the ECG complicated the diagnosis. Evaluation was further hindered by the known "variant pattern' seen on the ECGs of members of the Black population. We postulate that this patient's clinical features were a direct result of severe vasospasm affecting the left coronary artery; it is also strongly suggested that withdrawal from contact with industrial NG precipitated this potentially lethal coronary vasospasm. The role played by industrial NG in ischaemic heart disease is reviewed, as well as the importance of the "normal variant pattern' in the assessment of cardiac disease in Black patients. As far as we are aware this is the first time that the use of the ergonovine maleate provocative test has been documented in the industrial NG withdrawal syndrome.