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126-716: Zhuye Lake and the Hexi Corridor are located in the northern edge of the Qinghai-Tibetan Plateau , on the northern side of the Qilian Mountains , while in a transition zone between the Gobi Desert and highland. The area is in the northwest margin of the Asian summer monsoon. Some summer precipitation is from the summer monsoon moisture transport, while the area is mainly controlled by the mid-latitude westerly circulation, which

252-447: A direct bandgap material, recombination of these electron-hole pairs will result in cathodoluminescence; if the sample contains an internal electric field, such as is present at a p-n junction , the SEM beam injection of carriers will cause electron beam induced current (EBIC) to flow. Cathodoluminescence and EBIC are referred to as "beam-injection" techniques, and are very powerful probes of

378-424: A resin with further polishing to a mirror-like finish can be used for both biological and materials specimens when imaging in backscattered electrons or when doing quantitative X-ray microanalysis. The main preparation techniques are not required in the environmental SEM outlined below, but some biological specimens can benefit from fixation. Conventionally, a SEM specimen is required to be completely dry, since

504-406: A 3D image in real time. Other approaches use more sophisticated (and sometimes GPU-intensive) methods like the optimal estimation algorithm and offer much better results at the cost of high demands on computing power. In all instances, this approach works by integration of the slope, so vertical slopes and overhangs are ignored; for instance, if an entire sphere lies on a flat, little more than

630-412: A certain degree of complexity. Through the study of the modern process of regional climate on modern time scales, the interaction between monsoon and westerly winds is obvious, and this modern process can be used to imply the long-term interaction between the two climate systems. Paleoclimate simulations and lake records are two important means of past global change research. Simulation approach focuses on

756-474: A computer monitor (or, for vintage models, on a cathode-ray tube ). Each pixel of computer video memory is synchronized with the position of the beam on the specimen in the microscope, and the resulting image is, therefore, a distribution map of the intensity of the signal being emitted from the scanned area of the specimen. Older microscopes captured images on film, but most modern instruments collect digital images . Magnification in an SEM can be controlled over

882-436: A focused beam of electrons . The electrons interact with atoms in the sample, producing various signals that contain information about the surface topography and composition of the sample. The electron beam is scanned in a raster scan pattern, and the position of the beam is combined with the intensity of the detected signal to produce an image. In the most common SEM mode, secondary electrons emitted by atoms excited by

1008-446: A four-quadrant BSE detector (alternatively for one manufacturer, a 3-segment detector). The microscope produces four images of the same specimen at the same time, so no tilt of the sample is required. The method gives metrological 3D dimensions as far as the slope of the specimen remains reasonable. Most SEM manufacturers now (2018) offer such a built-in or optional four-quadrant BSE detector, together with proprietary software to calculate

1134-534: A higher speed than in any other part of the world. In the short term, this will cause lakes to expand and bring floods and mudflows. In the long run, the glaciers are vital lifelines for Asian rivers, including the Indus and the Ganges . Once they vanish, water supplies in those regions will be in peril. The Tibetan Plateau contains the largest area of low- latitude glaciers and is particularly vulnerable to global warming. Over

1260-512: A microscope with high resolution by scanning a very small raster with a demagnified and finely focused electron beam. In the same year, Cecil E. Hall also completed the construction of the first emission microscope in North America, just two years after being tasked by his supervisor, E. F. Burton at the University of Toronto. Ardenne applied scanning of the electron beam in an attempt to surpass

1386-489: A more natural rendering of the surface texture. Very often, published SEM images are artificially colored. This may be done for aesthetic effect, to clarify structure or to add a realistic appearance to the sample and generally does not add information about the specimen. Coloring may be performed manually with photo-editing software, or semi-automatically with dedicated software using feature-detection or object-oriented segmentation. In some configurations more information

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1512-399: A phosphor or scintillator positively biased to about +2,000 V. The accelerated secondary electrons are now sufficiently energetic to cause the scintillator to emit flashes of light (cathodoluminescence), which are conducted to a photomultiplier outside the SEM column via a light pipe and a window in the wall of the specimen chamber. The amplified electrical signal output by the photomultiplier

1638-427: A point resolution of 0.4 nm using a secondary electron detector. Conventional SEM requires samples to be imaged under vacuum , because a gas atmosphere rapidly spreads and attenuates electron beams. As a consequence, samples that produce a significant amount of vapour , e.g. wet biological samples or oil-bearing rock, must be either dried or cryogenically frozen. Processes involving phase transitions , such as

1764-652: A range of about 6 orders of magnitude from about 10 to 3,000,000 times. Unlike optical and transmission electron microscopes, image magnification in an SEM is not a function of the power of the objective lens . SEMs may have condenser and objective lenses, but their function is to focus the beam to a spot, and not to image the specimen. Provided the electron gun can generate a beam with a sufficiently small diameter, an SEM could in principle work entirely without condenser or objective lenses. However, it might not be very versatile or achieve very high resolution. In an SEM, as in scanning probe microscopy , magnification results from

1890-671: A result of this extremely inhospitable environment, the Changtang region (together with the adjoining Kekexili region) is the least populous region in Asia and the third least populous area in the world after Antarctica and northern Greenland. The geological history of the Tibetan Plateau is closely related to that of the Himalayas. The Himalayas belong to the Alpine Orogeny and are therefore among

2016-567: A specimen holder for viewing in the SEM. Integrated circuits may be cut with a focused ion beam (FIB) or other ion beam milling instrument for viewing in the SEM. The SEM in the first case may be incorporated into the FIB, enabling high-resolution imaging of the result of the process. Metals, geological specimens, and integrated circuits all may also be chemically polished for viewing in the SEM. Special high-resolution coating techniques are required for high-magnification imaging of inorganic thin films. In

2142-453: A spot about 0.4 nm to 5 nm in diameter. The beam passes through pairs of scanning coils or pairs of deflector plates in the electron column, typically in the final lens, which deflect the beam in the x and y axes so that it scans in a raster fashion over a rectangular area of the sample surface. When the primary electron beam interacts with the sample, the electrons lose energy by repeated random scattering and absorption within

2268-425: A standard tungsten filament SEM if the vacuum system is upgraded, or field emission guns (FEG), which may be of the cold-cathode type using tungsten single crystal emitters or the thermally assisted Schottky type, that use emitters of tungsten single crystals coated in zirconium oxide . The electron beam, which typically has an energy ranging from 0.2 keV to 40 keV, is focused by one or two condenser lenses to

2394-415: A steady wind to blow toward the land, which brings the moist air over the ocean surface with it. Rainfall is then increased by the presence of the moist ocean air. The rainfall is stimulated by a variety of mechanisms, such as low-level air being lifted upwards by mountains, surface heating, convergence at the surface, divergence aloft, or from storm-produced outflows near the surface. When such lifting occurs,

2520-436: A teardrop-shaped volume of the specimen known as the interaction volume , which extends from less than 100 nm to approximately 5 μm into the surface. The size of the interaction volume depends on the electron's landing energy, the atomic number of the specimen, and the specimen's density. The energy exchange between the electron beam and the sample results in the reflection of high-energy electrons by elastic scattering,

2646-621: A term usually referring to the localized, diurnal cycle of circulation near coastlines everywhere, but they are much larger in scale, stronger and seasonal. The seasonal monsoon wind shift and weather associated with the heating and cooling of the Tibetan plateau is the strongest such monsoon on Earth. Today, Tibet is an important heating surface of the atmosphere. However, during the Last Glacial Maximum , an approximately 2,400,000 square kilometres (930,000 sq mi) ice sheet covered

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2772-471: A typical SEM, an electron beam is thermionically emitted from an electron gun fitted with a tungsten filament cathode . Tungsten is normally used in thermionic electron guns because it has the highest melting point and lowest vapor pressure of all metals, thereby allowing it to be electrically heated for electron emission, and because of its low cost. Other types of electron emitters include lanthanum hexaboride ( LaB 6 ) cathodes, which can be used in

2898-426: A variable pressure or environmental SEM, and at a wide range of cryogenic or elevated temperatures with specialized instruments. An account of the early history of scanning electron microscopy has been presented by McMullan. Although Max Knoll produced a photo with a 50 mm object-field-width showing channeling contrast by the use of an electron beam scanner, it was Manfred von Ardenne who in 1937 invented

3024-517: A well-defined, three-dimensional appearance. Using the signal of secondary electrons image resolution less than 0.5 nm is possible. Backscattered electrons (BSE) consist of high-energy electrons originating in the electron beam, that are reflected or back-scattered out of the specimen interaction volume by elastic scattering interactions with specimen atoms. Since heavy elements (high atomic number) backscatter electrons more strongly than light elements (low atomic number), and thus appear brighter in

3150-433: Is better captured by the secondary electrons detector and combine it to the information about density, obtained by the backscattered electron detector. Measurement of the energy of photons emitted from the specimen is a common method to get analytical capabilities. Examples are the energy-dispersive X-ray spectroscopy (EDS) detectors used in elemental analysis and cathodoluminescence microscope (CL) systems that analyse

3276-409: Is a key area for studies on the interactions between the summer monsoon and the westerlies. In recent years, scientists have made a lot of progress on the late Quaternary environmental changes in the area, and promoted the study of long-term monsoon and westerly winds changes, but there are still some controversies. A major problem is that there are a large volume of papers in the area that are related to

3402-479: Is also applicable to the imaging of temperature-sensitive materials such as ice and fats. Freeze-fracturing, freeze-etch or freeze-and-break is a preparation method particularly useful for examining lipid membranes and their incorporated proteins in "face on" view. The preparation method reveals the proteins embedded in the lipid bilayer. Back-scattered electron imaging, quantitative X-ray analysis, and X-ray mapping of specimens often requires grinding and polishing

3528-400: Is capable of producing high primary electron brightness and small spot size even at low accelerating potentials. To prevent charging of non-conductive specimens, operating conditions must be adjusted such that the incoming beam current is equal to sum of outgoing secondary and backscattered electron currents, a condition that is most often met at accelerating voltages of 0.3–4 kV. Embedding in

3654-417: Is displayed as a two-dimensional intensity distribution that can be viewed and photographed on an analogue video display, or subjected to analog-to-digital conversion and displayed and saved as a digital image . This process relies on a raster-scanned primary beam. The brightness of the signal depends on the number of secondary electrons reaching the detector . If the beam enters the sample perpendicular to

3780-409: Is distributed vertically through a "mixed layer" that may be 50 meters deep through the action of wind and buoyancy-generated turbulence , whereas the land surface conducts heat slowly, with the seasonal signal penetrating only a meter or so. Additionally, the specific heat capacity of liquid water is significantly greater than that of most materials that make up land. Together, these factors mean that

3906-450: Is enhanced. An alternative to coating for some biological samples is to increase the bulk conductivity of the material by impregnation with osmium using variants of the OTO staining method (O- osmium tetroxide , T- thiocarbohydrazide , O- osmium ). Nonconducting specimens may be imaged without coating using an environmental SEM (ESEM) or low-voltage mode of SEM operation. In ESEM instruments

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4032-468: Is gathered per pixel, often by the use of multiple detectors. As a common example, secondary electron and backscattered electron detectors are superimposed and a color is assigned to each of the images captured by each detector, with a result of a combined color image where colors are related to the density of the components. This method is known as density-dependent color SEM (DDC-SEM). Micrographs produced by DDC-SEM retain topographical information, which

4158-441: Is known as false color . On a BSE image, false color may be performed to better distinguish the various phases of the sample. As an alternative to simply replacing each grey level by a color, a sample observed by an oblique beam allows researchers to create an approximative topography image (see further section "Photometric 3D rendering from a single SEM image" ). Such topography can then be processed by 3D-rendering algorithms for

4284-427: Is present within the sample during drying. The dry specimen is usually mounted on a specimen stub using an adhesive such as epoxy resin or electrically conductive double-sided adhesive tape, and sputter-coated with gold or gold/palladium alloy before examination in the microscope. Samples may be sectioned (with a microtome ) if information about the organism's internal ultrastructure is to be exposed for imaging. If

4410-496: Is produced by collecting back-scattered electrons from one side above the specimen using an asymmetrical, directional BSE detector; the resulting contrast appears as illumination of the topography from that side. Semiconductor detectors can be made in radial segments that can be switched in or out to control the type of contrast produced and its directionality. Backscattered electrons can also be used to form an electron backscatter diffraction (EBSD) image that can be used to determine

4536-452: Is rare for a single machine to have detectors for all other possible signals. Secondary electrons have very low energies on the order of 50 eV , which limits their mean free path in solid matter. Consequently, SEs can only escape from the top few nanometers of the surface of a sample. The signal from secondary electrons tends to be highly localized at the point of impact of the primary electron beam, making it possible to collect images of

4662-612: Is rich in carbonate minerals, forming a huge inorganic carbon pool. SEM analysis of quartz sands reveals that sand sediments are from diverse sources in the region. Sands in the lake have experienced both the aeolian and the lacustrine deposition processes, indicating the complicated formation processes of those sand layers. Total organic carbon (TOC), carbon and nitrogen ratio (C/N) and organic carbon isotope (δ13Corg) are commonly used in lake sediments for paleoclimate reconstruction, which are indicators of basin-wide primary productivity and vegetation types in lakes and watersheds, as well as

4788-776: Is the Qilian Mountains. The line of mountains continues east of the plateau as the Qinling , which separates the Ordos Plateau from Sichuan. North of the mountains runs the Gansu or Hexi Corridor which was the main silk-road route from China proper to the West. The plateau is a high-altitude arid steppe interspersed with mountain ranges and large brackish lakes. Annual precipitation ranges from 100 to 300 millimetres (3.9 to 11.8 in) and falls mainly as hail . The southern and eastern edges of

4914-519: Is the world's highest and largest plateau above sea level, with an area of 2,500,000 square kilometres (970,000 sq mi). With an average elevation exceeding 4,500 metres (14,800 ft) and being surrounded by imposing mountain ranges that harbor the world's two highest summits, Mount Everest and K2 , the Tibetan Plateau is often referred to as "the Roof of the World ". The Tibetan Plateau contains

5040-421: Is then dehydrated. Because air-drying causes collapse and shrinkage, this is commonly achieved by replacement of water in the cells with organic solvents such as ethanol or acetone , and replacement of these solvents in turn with a transitional fluid such as liquid carbon dioxide by critical point drying . The carbon dioxide is finally removed while in a supercritical state, so that no gas–liquid interface

5166-489: Is used extensively for defect analysis of semiconductor wafers , and manufacturers make instruments that can examine any part of a 300 mm semiconductor wafer. Many instruments have chambers that can tilt an object of that size to 45° and provide continuous 360° rotation. Nonconductive specimens collect charge when scanned by the electron beam, and especially in secondary electron imaging mode, this causes scanning faults and other image artifacts. For conventional imaging in

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5292-512: Is useful because coating can be difficult to reverse, may conceal small features on the surface of the sample and may reduce the value of the results obtained. X-ray analysis is difficult with a coating of a heavy metal, so carbon coatings are routinely used in conventional SEMs, but ESEM makes it possible to perform X-ray microanalysis on uncoated non-conductive specimens; however some specific for ESEM artifacts are introduced in X-ray analysis. ESEM may be

5418-549: The Changtang , is generally too high and cold to support permanent population. One of the most notable civilizations to have developed on the Tibetan Plateau is the Tibetan Empire from the 7th century to the 9th century AD. Monsoons are caused by the different amplitudes of surface temperature seasonal cycles between land and oceans. This differential warming occurs because heating rates differ between land and water. Ocean heating

5544-525: The Eurasian Plate . About 50 million years ago, this fast-moving Indo-Australian plate had completely closed the Tethys Ocean , the existence of which has been determined by sedimentary rocks settled on the ocean floor, and the volcanoes that fringed its edges. Since these sediments were light, they crumpled into mountain ranges rather than sinking to the floor. During this early stage of its formation in

5670-922: The Kunlun Mountains separate the plateau from the Tarim Basin. About halfway across the Tarim the bounding range becomes the Altyn-Tagh and the Kunluns, by convention, continue somewhat to the south. In the 'V' formed by this split is the western part of the Qaidam Basin . The Altyn-Tagh ends near the Dangjin pass on the Dunhuang – Golmud road. To the west are short ranges called the Danghe, Yema, Shule, and Tulai Nanshans. The easternmost range

5796-422: The Last Glacial Maximum (LGM). The Zhuye Lake record is a typical record in monsoon marginal zones, which matches well with the simulation results, and the simulation method further explains the mechanism of the millennial-scale lake evolution. Qinghai-Tibetan Plateau The Tibetan Plateau , also known as Qinghai–Tibet Plateau and Qing–Zang Plateau , is a vast elevated plateau located at

5922-452: The Quaternary science while various subjects are involved. That is not conducive for scholars to understand the late Quaternary conditions in the area. The reference guide is designed to organize literatures in relevant disciplines, which is good for scientists to understand the late Quaternary environments easily, while the guide also represents the research progress of Lanzhou University on

6048-563: The headwaters of the drainage basins of most of the streams and rivers in surrounding regions . This includes the three longest rivers in Asia (the Yellow , Yangtze , and Mekong ). Its tens of thousands of glaciers and other geographical and ecological features serve as a " water tower " storing water and maintaining flow . It is sometimes termed the Third Pole because its ice fields contain

6174-464: The resolution is not limited by the diffraction limit , fineness of lenses or mirrors or detector array resolution. The focusing optics can be large and coarse, and the SE detector is fist-sized and simply detects current. Instead, the spatial resolution of the SEM depends on the size of the electron spot, which in turn depends on both the wavelength of the electrons and the electron-optical system that produces

6300-720: The thermal low pressure caused by the heating, there was no monsoon over the Indian subcontinent . This lack of monsoon caused extensive rainfall over the Sahara , expansion of the Thar Desert , more dust deposited into the Arabian Sea , and a lowering of the biotic life zones on the Indian subcontinent. Animals responded to this shift in climate, with the Javan rusa migrating into India. In addition,

6426-402: The (x, y) pixel position. This single number is usually represented, for each pixel, by a grey level, forming a monochrome image. However, several ways have been used to get color electron microscopy images. The easiest way to get color is to associate to this single number an arbitrary color, using a color look-up table (i.e. each grey level is replaced by a chosen color). This method

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6552-704: The 1950s and early 1960s headed by Charles Oatley , all of which finally led to the marketing of the first commercial instrument by Cambridge Scientific Instrument Company as the "Stereoscan" in 1965, which was delivered to DuPont . The signals used by a SEM to produce an image result from interactions of the electron beam with atoms at various depths within the sample. Various types of signals are produced including secondary electrons (SE), reflected or back-scattered electrons (BSE), characteristic X-rays and light ( cathodoluminescence ) (CL), absorbed current (specimen current) and transmitted electrons. Secondary electron detectors are standard equipment in all SEMs, but it

6678-638: The Chinese lowlands. Ice of the plateau provides a valuable window to the past. In 2015, researchers studying the Plateau reached the top of the Guliya glacier , with ice thickness of 310 m (1,020 ft), and drilled to a depth of 50 m (160 ft) in order to recover ice core samples. Due to the extremely low biomass in those 15,000-year-old samples, it had taken around 5 years of research to extract 33 viruses, of which 28 were new to science. None had survived

6804-472: The ESEM in the late 1980s allowed samples to be observed in low-pressure gaseous environments (e.g. 1–50 Torr or 0.1–6.7 kPa) and high relative humidity (up to 100%). This was made possible by the development of a secondary-electron detector capable of operating in the presence of water vapour and by the use of pressure-limiting apertures with differential pumping in the path of the electron beam to separate

6930-466: The Late Glacial and early Holocene and a lake retreat during the late Holocene. This change shows the effects of the Asian summer monsoon on the millennial-scale and variations of the northern boundary of the Asian summer monsoon. Millennium-scale environmental changes recorded in the sediments of Huahai Lake showed that basin-wide runoff was high while erosion and deposition processes were intense during

7056-874: The Late Palaeogene, Tibet consisted of a deep palaeovalley bounded by multiple mountain ranges rather than the more topographically uniform elevated flatland that it is today. The Tibetan Plateau's mean elevation continued to vary since its initial uplift in the Eocene; isotopic records show the plateau's altitude was around 3,000 metres above sea level around the Oligocene-Miocene boundary and that it fell by 900 metres between 25.5 and 21.6 million years ago, attributable to tectonic unroofing from east–west extension or to erosion from climatic weathering. The plateau subsequently rose by 500 to 1,000 metres between 21.6 and 20.4 million years ago. Palaeobotanical evidence indicates that

7182-739: The Nujiang Suture Zone and the Yarlung-Zangpo Suture Zone remained tropical or subtropical lowlands until the latest Oligocene or Early Miocene , enabling biotic interchange across Tibet. The age of east–west grabens in the Lhasa and Himalaya terranes suggests that the plateau's elevation was close to its modern altitude by around 14 to 8 million years ago. Erosion rates in Tibet decreased significantly around 10 million years ago. The Indo-Australian plate continues to be driven horizontally below

7308-422: The SEM is equipped with a cold stage for cryo microscopy, cryofixation may be used and low-temperature scanning electron microscopy performed on the cryogenically fixed specimens. Cryo-fixed specimens may be cryo-fractured under vacuum in a special apparatus to reveal internal structure, sputter-coated and transferred onto the SEM cryo-stage while still frozen. Low-temperature scanning electron microscopy (LT-SEM)

7434-404: The SEM, specimens must be electrically conductive , at least at the surface, and electrically grounded to prevent the accumulation of electrostatic charge . Metal objects require little special preparation for SEM except for cleaning and conductively mounting to a specimen stub. Non-conducting materials are usually coated with an ultrathin coating of electrically conducting material, deposited on

7560-428: The Tibetan Plateau, which forces the plateau to move upwards; the plateau is still rising at a rate of approximately 5 mm (0.2 in) per year (although erosion reduces the actual increase in height). Much of the Tibetan Plateau is of relatively low relief. The cause of this is debated among geologists. Some argue that the Tibetan Plateau is an uplifted peneplain formed at low altitude, while others argue that

7686-523: The Tibetan plateau from around 200,000 to 40,000 years ago, according to a study published in Nature . Nomads on the Tibetan Plateau and in the Himalayas are the remainders of nomadic practices historically once widespread in Asia and Africa. Pastoral nomads constitute about 40% of the ethnic Tibetan population. The presence of nomadic peoples on the plateau is predicated on their adaptation to survival on

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7812-431: The ability to image a comparatively large area of the specimen; the ability to image bulk materials (not just thin films or foils); and the variety of analytical modes available for measuring the composition and properties of the specimen. Depending on the instrument, the resolution can fall somewhere between less than 1 nm and 20 nm. As of 2009, The world's highest resolution conventional (≤30 kV) SEM can reach

7938-433: The air cools due to expansion in lower pressure, which in turn produces condensation and precipitation. In winter, the land cools off quickly, but the ocean maintains the heat longer. The hot air over the ocean rises, creating a low-pressure area and a breeze from land to ocean while a large area of drying high pressure is formed over the land, increased by wintertime cooling. Monsoons are similar to sea and land breezes ,

8064-406: The basin-wide precipitation is the main controlling factor, but the lake evaporation and the basin-wide evapotranspiration also affect millennial-scale changes of lake area. Modern process of climate change is the basis for the study of paleoclimate change. The northwest margin of the Asian summer monsoon is influenced by both the monsoon and the westerly winds, where the paleoclimate change reflects

8190-591: The characteristic X-rays, because the intensity of the BSE signal is strongly related to the atomic number (Z) of the specimen. BSE images can provide information about the distribution, but not the identity, of different elements in the sample. In samples predominantly composed of light elements, such as biological specimens, BSE imaging can image colloidal gold immuno-labels of 5 or 10 nm diameter, which would otherwise be difficult or impossible to detect in secondary electron images. Characteristic X-rays are emitted when

8316-411: The crime scene, victim, or shooter and analyzed with the SEM. This can help scientists determine proximity and or contact with the discharged firearm. Electron microscopes do not naturally produce color images, as an SEM produces a single value per pixel ; this value corresponds to the number of electrons received by the detector during a small period of time of the scanning when the beam is targeted to

8442-440: The crystallographic structure of the specimen. The nature of the SEM's probe, energetic electrons, makes it uniquely suited to examining the optical and electronic properties of semiconductor materials. The high-energy electrons from the SEM beam will inject charge carriers into the semiconductor. Thus, beam electrons lose energy by promoting electrons from the valence band into the conduction band , leaving behind holes . In

8568-644: The dating theories, the two methods are different. Further studies are still needed for a comparison between the two methods. Atmospheric circulation features and the evaporation play important roles in millennial-scale lake evolution. Lake evaporation is mainly affected by relative humidity, temperature, vapor pressure and sunshine duration. According to modern observations, temperature and relative humidity have important roles in Holocene millennial-scale lake level changes at Zhuye Lake. Further studies have shown that lake level changes are mainly controlled by many factors, and

8694-483: The difference in instrumentation, this technique is still commonly referred to as scanning transmission electron microscopy (STEM) . The SEM is used often in Forensic Science for magnified analysis of microscopic things such as diatoms and gunshot residue . Because SEM is a nondestructive force on the sample, it can be used to analyze evidence without damaging it. The SEM shoots a beam of high energy electrons to

8820-534: The drying of adhesives or melting of alloys , liquid transport, chemical reactions, and solid-air-gas systems, in general cannot be observed with conventional high-vacuum SEM. In environmental SEM (ESEM), the chamber is evacuated of air, but water vapor is retained near its saturation pressure, and the residual pressure remains relatively high. This allows the analysis of samples containing water or other volatile substances. With ESEM, observations of living insects have been possible. The first commercial development of

8946-505: The early Holocene, the highest primary productivity in the mid-Holocene and an arid trend in late Holocene. Height measurements and OSL/14C dating of paleo-shorelines in Zhuye Lake can better understand the formation and evolution of lake levels during the late Quaternary. According to field surveys, there are 9 paleo-shorelines and one lake terrace in the northeast of Zhuye Lake. OSL and 14C ages have showed that high lake levels existed during

9072-422: The early Holocene. The effective moisture during the mid-Holocene was still high, then the late Holocene was relatively dry. This change in water conditions was related to the strong early-to-mid Holocene Asian summer monsoon that brings more rainfall to this area. As a result, the runoff was high and the erosion was intensive correspondingly. This changing pattern of Holocene moisture conditions were also affected by

9198-501: The early and middle Holocene, and the lake retreated significantly during the late Holocene. On the Holocene millennial-scale, there are some differences between OSL ages and 14C ages on those paleo-shorelines. However, they commonly show the high lake level periods during the early and middle Holocene. Major differences between the two dating methods appeared in ages of the MIS3 and the MIS5. Regarding

9324-400: The electron beam are detected using a secondary electron detector ( Everhart–Thornley detector ). The number of secondary electrons that can be detected, and thus the signal intensity, depends, among other things, on specimen topography. Some SEMs can achieve resolutions better than 1 nanometer. Specimens are observed in high vacuum in a conventional SEM, or in low vacuum or wet conditions in

9450-411: The electron beam removes an inner shell electron from the sample, causing a higher-energy electron to fill the shell and release energy. The energy or wavelength of these characteristic X-rays can be measured by Energy-dispersive X-ray spectroscopy or Wavelength-dispersive X-ray spectroscopy and used to identify and measure the abundance of elements in the sample and map their distribution. Due to

9576-581: The elevation and low precipitation. The Tibetan Plateau hosts the Tibetan wolf , and species of snow leopard , wild yak , wild ass , cranes, vultures, hawks, geese, snakes, and water buffalo . One notable animal is the high-altitude jumping spider , that can live at elevations of over 6,500 metres (21,300 ft). Ecoregions found on the Tibetan Plateau, as defined by the World Wide Fund for Nature , are as follows: Extinct humans ( Denisovans ) lived on

9702-422: The emission of secondary electrons by inelastic scattering , and the emission of electromagnetic radiation , each of which can be detected by specialized detectors. The beam current absorbed by the specimen can also be detected and used to create images of the distribution of specimen current. Electronic amplifiers of various types are used to amplify the signals, which are displayed as variations in brightness on

9828-583: The extraction process. Phylogenetic analysis suggests those viruses infected plants or other microorganisms. The Tibetan Plateau contains the world's third-largest store of ice. Qin Dahe, the former head of the China Meteorological Administration , issued the following assessment in 2009: Temperatures are rising four times faster than elsewhere in China, and the Tibetan glaciers are retreating at

9954-476: The glaciers in Tibet created meltwater lakes in the Qaidam Basin , the Tarim Basin , and the Gobi Desert , despite the strong evaporation caused by the low latitude. Silt and clay from the glaciers accumulated in these lakes; when the lakes dried at the end of the ice age, the silt and clay were blown by the downslope wind off the Plateau. These airborne fine grains produced the enormous amount of loess in

10080-400: The heat capacity of the layer participating in the seasonal cycle is much larger over the oceans than over land, with the consequence that the land warms and cools faster than the ocean. In turn, air over the land warms faster and reaches a higher temperature than does air over the ocean. The warmer air over land tends to rise, creating an area of low pressure . The pressure anomaly then causes

10206-448: The higher energy BSE. Dedicated backscattered electron detectors are positioned above the sample in a "doughnut" type arrangement, concentric with the electron beam, maximizing the solid angle of collection. BSE detectors are usually either of scintillator or of semiconductor types. When all parts of the detector are used to collect electrons symmetrically about the beam, atomic number contrast is produced. However, strong topographic contrast

10332-410: The image, BSEs are used to detect contrast between areas with different chemical compositions. The Everhart–Thornley detector, which is normally positioned to one side of the specimen, is inefficient for the detection of backscattered electrons because few such electrons are emitted in the solid angle subtended by the detector, and because the positively biased detection grid has little ability to attract

10458-416: The intensity and spectrum of electron-induced luminescence in (for example) geological specimens. In SEM systems using these detectors it is common to color code these extra signals and superimpose them in a single color image, so that differences in the distribution of the various components of the specimen can be seen clearly and compared. Optionally, the standard secondary electron image can be merged with

10584-480: The interaction between the summer monsoon and the westerly winds. Chronologies of lake sediments in Zhuye Lake were mainly from pollen concentrates, bulk organic matter, bulk inorganic matter and mollusk shells AMS 14C and conventional 14C dating, as well as OSL dating. A comparison between different dating methods and results shows that the reservoir effect was relatively weak during the Holocene. However, inverted ages also exist in some sedimentary layers, especially in

10710-433: The interaction of electrons with the sample may also be detected in an SEM equipped for energy-dispersive X-ray spectroscopy or wavelength dispersive X-ray spectroscopy . Analysis of the x-ray signals may be used to map the distribution and estimate the abundance of elements in the sample. An SEM is not a camera and the detector is not continuously image-forming like a CCD array or film . Unlike in an optical system,

10836-884: The intersection of Central , South , and East Asia . . Geographically, it is located to the north of Himalayas and the Indian subcontinent , and to the south of Tarim Basin and Mongolian Plateau . Geopolitically, it covers most of the Tibet Autonomous Region , most of Qinghai , western half of Sichuan , Southern Gansu provinces in Western China , southern Xinjiang , Bhutan , the Indian regions of Ladakh and Lahaul and Spiti ( Himachal Pradesh ) as well as Gilgit-Baltistan in Pakistan , northwestern Nepal , eastern Tajikistan and southern Kyrgyzstan . It stretches approximately 1,000 kilometres (620 mi) north to south and 2,500 kilometres (1,600 mi) east to west. It

10962-441: The lake and watershed reliably. Lake sediment lithology, minerals and SEM ( Scanning Electron Microscopy ) analysis of quartz sands are also good indicators reflecting the evolution of the lake in arid regions. Lacustrine sediments in Zhuye Lake are mainly characterized by grey silts. Some lake sediment layers are mixed with peat or rust brown layers, while grey, brown or yellow sand layers are also embedded between lake sediments. On

11088-464: The lake basin. Environmental changes in Zhuye Lake are mainly affected by the evolution of the Asian summer monsoon, but also affected by the mid-latitude westerlies, reflecting the interaction of the two large circulation systems. The Yanchi Lake record is consistent with paleoclimate records from typical monsoon domain and the Qinghai-Tibet Plateau, which indicates the lake expansion during

11214-474: The lake sediment proxies from various sites of the lake basin. Along with millennial-scale environmental and ecological changes, pollen records in Zhuye Lake reflected basin-wide vegetation history well. However, affected by the complicated topography of the lake basin, there are some differences in pollen assemblages from different sites of the lake basin. Pollen records from the western lake were significantly affected by alluvial pollen whereas pollen records from

11340-405: The largest reserve of fresh water outside the polar regions. The impact of climate change on the Tibetan Plateau is of ongoing scientific interest. The Tibetan Plateau is surrounded by the massive mountain ranges of high-mountain Asia . The plateau is bordered to the south by the inner Himalayan range , to the north by the Kunlun Mountains , which separate it from the Tarim Basin , and to

11466-400: The late Quaternary environments. Lake records from Zhuye Lake show that early Holocene was relatively humid and the mid-Holocene environment was characterized by an appropriate combination of precipitation and evaporation. An arid trend appeared during the late Holocene. There was a centennial-scale drought event during the mid-Holocene, which was recorded by sand layers in various locations of

11592-404: The latter, the plateau is affected by distributed deformation resulting from flow within the crust. The Tibetan Plateau supports a variety of ecosystems, most of them classified as montane grasslands. While parts of the plateau feature an alpine tundra -like environment, other areas feature monsoon-influenced shrublands and forests. Species diversity is generally reduced on the plateau due to

11718-428: The low relief stems from erosion and infill of topographic depressions that occurred at already high elevations. The current tectonics of the plateau are also debated. The best-regarded explanations are provided by the block model and the alternative continuum model. According to the former, the crust of the plateau is formed of several blocks with little internal deformation separated by major strike-slip faults . In

11844-415: The magnification limit of the best light microscopes . SEM samples have to be small enough to fit on the specimen stage, and may need special preparation to increase their electrical conductivity and to stabilize them, so that they can withstand the high vacuum conditions and the high energy beam of electrons. Samples are generally mounted rigidly on a specimen holder or stub using a conductive adhesive. SEM

11970-742: The mid-latitude westerlies to some extent. Paleoenvironmental proxies in lake sediments from Zhuye Lake show that there are distinct ~256 ~512 ~1024 climate cycles, which are consistent with millennial-scale and centennial-scale climate cycles in the typical Asian summer monsoon domain during the Holocene. Meanwhile, lake sediments in some sites of the lake basin also recorded the Holocene North Atlantic Ice Debris Events (Bond Events). These records from monsoon marginal zones reflected regional responses to global scale Holocene climate cycles. Mid-Holocene drought events were widely recovered from lake sediments in monsoon marginal zones, but

12096-476: The middle and the eastern lake reflect pollen assemblages well mixed by lake hydrodynamic processes. Overall, tree pollen is relatively high during the early Holocene due to high runoff and precipitation. Pollen concentration was high and pollen types were rich during the mid-Holocene. A late Holocene dry tendency was also recorded by pollen assemblages in the lake. Pollen records in Zhuye Lake are consistent with other proxy records and indicate environmental changes in

12222-602: The northeast by the Qilian Mountains , which separate the plateau from the Hexi Corridor and Gobi Desert . To the east and southeast the plateau gives way to the forested gorge and ridge geography of the mountainous headwaters of the Salween , Mekong , and Yangtze rivers in northwest Yunnan and western Sichuan (the Hengduan Mountains ). In the west, the curve of the rugged Karakoram range of northern Kashmir embraces

12348-401: The one or more compositional channels, so that the specimen's structure and composition can be compared. Such images can be made while maintaining the full integrity of the original signal data, which is not modified in any way. SEMs do not naturally provide 3D images contrary to SPMs . However 3D data can be obtained using an SEM with different methods as follows. This method typically uses

12474-405: The optoelectronic behavior of semiconductors, in particular for studying nanoscale features and defects. Cathodoluminescence , the emission of light when atoms excited by high-energy electrons return to their ground state, is analogous to UV -induced fluorescence , and some materials such as zinc sulfide and some fluorescent dyes, exhibit both phenomena. Over the last decades, cathodoluminescence

12600-403: The past five decades, 80% of the glaciers in the Tibetan Plateau have retreated, losing 4.5% of their combined areal coverage. This region is also liable to suffer damages from permafrost thaw caused by climate change. Scanning Electron Microscopy A scanning electron microscope ( SEM ) is a type of electron microscope that produces images of a sample by scanning the surface with

12726-513: The person died. By using the images produced by the SEM, forensic scientists can compare diatoms types to confirm the body of water a person died in. Gunshot residue (GSR) analysis can be done with many different analytical instruments, but SEM is a common way to analyze inorganic compounds because of the way it can closely analyze the types of elements (mostly metals) through its three detectors: backscatter electron detector, secondary electron detector, and X-ray detector . GSR can be collected from

12852-507: The plateau. Due to its great extent, this glaciation in the subtropics was an important element of radiative forcing . With a much lower latitude, the ice in Tibet reflected at least four times more radiation energy per unit area into space than ice at higher latitudes . Thus, while the modern plateau heats the overlying atmosphere, during the Last Ice Age it helped to cool it. This cooling had multiple effects on regional climate. Without

12978-406: The plateau. The Indus River originates in the western Tibetan Plateau in the vicinity of Lake Manasarovar . The Tibetan Plateau is bounded in the north by a broad escarpment where the altitude drops from around 5,000 metres (16,000 ft) to 1,500 metres (4,900 ft) over a horizontal distance of less than 150 kilometres (93 mi). Along the escarpment is a range of mountains. In the west,

13104-551: The preferred for electron microscopy of unique samples from criminal or civil actions, where forensic analysis may need to be repeated by several different experts. It is possible to study specimens in liquid with ESEM or with other liquid-phase electron microscopy methods. The SEM can also be used in transmission mode by simply incorporating an appropriate detector below a thin specimen section. Detectors are available for bright field, dark field, as well as segmented detectors for mid-field to high angle annular dark-field . Despite

13230-447: The preservation of organic matter. Changes in TOC and C/N have a similar trend and their high values indicate high basin-wide primary productivity. Changes in δ13Corg are negatively correlated with other two proxies, while its lower values indicate high effective moisture. Lake sediment geochemical proxies correspond well with other sediment proxies, indicating high precipitation and runoff during

13356-554: The ratio of the raster on the display device and dimensions of the raster on the specimen. Assuming that the display screen has a fixed size, higher magnification results from reducing the size of the raster on the specimen, and vice versa. Magnification is therefore controlled by the current supplied to the x, y scanning coils, or the voltage supplied to the x, y deflector plates, and not by objective lens power. The most common imaging mode collects low-energy (<50 eV) secondary electrons that are ejected from conduction or valence bands of

13482-509: The resolution of the transmission electron microscope (TEM), as well as to mitigate substantial problems with chromatic aberration inherent to real imaging in the TEM. He further discussed the various detection modes, possibilities and theory of SEM, together with the construction of the first high resolution SEM . Further work was reported by Zworykin's group, followed by the Cambridge groups in

13608-456: The sample either by low-vacuum sputter coating , electroless deposition or by high-vacuum evaporation. Conductive materials in current use for specimen coating include gold , gold/ palladium alloy, platinum , iridium , tungsten , chromium , osmium , and graphite . Coating with heavy metals may increase signal/noise ratio for samples of low atomic number (Z). The improvement arises because secondary electron emission for high-Z materials

13734-414: The sample surface with a resolution of below 1 nm . Back-scattered electrons (BSE) are beam electrons that are reflected from the sample by elastic scattering . Since they have much higher energy than SEs, they emerge from deeper locations within the specimen and, consequently, the resolution of BSE images is less than SE images. However, BSE are often used in analytical SEM, along with the spectra made from

13860-408: The sample which bounce off of the sample without changing or destroying it. This is great when it comes to analyzing diatoms. When a person dies by drowning, they inhale the water which causes what is in the water (diatoms) to get in the blood stream, brain, kidneys, and more. These diatoms in the body can be magnified with the SEM to determine the type of diatoms which aid in understanding how and where

13986-450: The scanning beam. The resolution is also limited by the size of the interaction volume, the volume of specimen material that interacts with the electron beam. The spot size and the interaction volume are both large compared to the distances between atoms, so the resolution of the SEM is not high enough to image individual atoms, as is possible with a transmission electron microscope (TEM). The SEM has compensating advantages, though, including

14112-472: The specimen atoms by inelastic scattering interactions with beam electrons. Due to their low energy, these electrons originate from within a few nanometers below the sample surface. The electrons are detected by an Everhart–Thornley detector , which is a type of collector- scintillator - photomultiplier system. The secondary electrons are first collected by attracting them towards an electrically biased grid at about +400 V, and then further accelerated towards

14238-593: The specimen chamber is under high vacuum. Hard, dry materials such as wood, bone, feathers, dried insects, or shells (including egg shells ) can be examined with little further treatment, but living cells and tissues and whole, soft-bodied organisms require chemical fixation to preserve and stabilize their structure. Fixation is usually performed by incubation in a solution of a buffered chemical fixative, such as glutaraldehyde , sometimes in combination with formaldehyde and other fixatives, and optionally followed by postfixation with osmium tetroxide. The fixed tissue

14364-405: The specimen is placed in a relatively high-pressure chamber and the electron optical column is differentially pumped to keep vacuum adequately low at the electron gun. The high-pressure region around the sample in the ESEM neutralizes charge and provides an amplification of the secondary electron signal. Low-voltage SEM is typically conducted in an instrument with a field emission guns (FEG) which

14490-511: The steppe have grasslands that can sustainably support populations of nomadic herdsmen, although frost occurs for six months of the year. Permafrost occurs over extensive parts of the plateau. Proceeding to the north and northwest, the plateau becomes progressively higher, colder, and drier, until reaching the remote Changtang region in the northwestern part of the plateau. Here the average altitude exceeds 5,000 metres (16,000 ft) and winter temperatures can drop to −40 °C (−40 °F). As

14616-450: The strata of the Late Glacial and early Holocene. The disordered ages in those sedimentary layers were mainly affected by the reworking effects. According to the OSL and radiocarbon ages from paleo-shorelines in the northeast of the lake basin, high lake levels mainly occurred during the early and middle Holocene, and a drought tendency appeared during the late Holocene. The result is consistent with

14742-418: The study of paleoclimate change mechanism, and the lake record is mainly used for paleoclimate reconstruction. The combination of the two methods is an important way to understand paleoclimate change and long-term water cycle mechanisms. According to CCSM 3.0 paleoclimate models, lake water & energy balance models, paleoclimate simulation results match well with the lake records in monsoon marginal zones since

14868-442: The surface, then the activated region is uniform about the axis of the beam and a certain number of electrons "escape" from within the sample. As the angle of incidence increases, the interaction volume increases and the "escape" distance of one side of the beam decreases, resulting in more secondary electrons being emitted from the sample. Thus steep surfaces and edges tend to be brighter than flat surfaces, which results in images with

14994-454: The surfaces to an ultra-smooth surface. Specimens that undergo WDS or EDS analysis are often carbon-coated. In general, metals are not coated prior to imaging in the SEM because they are conductive and provide their own pathway to ground. Fractography is the study of fractured surfaces that can be done on a light microscope or, commonly, on an SEM. The fractured surface is cut to a suitable size, cleaned of any organic residues, and mounted on

15120-458: The time scales and mechanisms are still in dispute. According to lithology, chronology and paleoclimate proxies, this drought event occurred mainly between ~ 8.0-~ 7.0 cal kyr BP and its influence was limited in the middle and lower reaches of the Shiyang River whereas the upper reaches were less affected. The drought event could be primarily controlled by changes in the ratio of water and heat and

15246-502: The top of the sediment sections in the lake, aeolian sediments with uneven thickness are deposited, which is an indicative of the lake retreat during the late Holocene. These changes in lithology can be linked to lake hydrodynamic conditions and sand activities in the area. Carbonate mineral is the major salt mineral in the lake. Millennial-scale evolution of salt minerals reflects the general rules of Salt Lake, and changes in salt minerals correspond with variations in other proxies. The region

15372-608: The vacuum region (around the gun and lenses) from the sample chamber. The first commercial ESEMs were produced by the ElectroScan Corporation in USA in 1988. ElectroScan was taken over by Philips (who later sold their electron-optics division to FEI Company) in 1996. ESEM is especially useful for non-metallic and biological materials because coating with carbon or gold is unnecessary. Uncoated plastics and elastomers can be routinely examined, as can uncoated biological samples. This

15498-412: The very narrow electron beam, SEM micrographs have a large depth of field yielding a characteristic three-dimensional appearance useful for understanding the surface structure of a sample. This is exemplified by the micrograph of pollen shown above. A wide range of magnifications is possible, from about 10 times (about equivalent to that of a powerful hand-lens) to more than 500,000 times, about 250 times

15624-400: The world's grassland by raising livestock rather than crops, which are unsuitable to the terrain. Archaeological evidence suggests that the earliest human occupation of the plateau occurred between 30,000 and 40,000 years ago. Since colonization of the Tibetan Plateau, Tibetan culture has adapted and flourished in the western, southern, and eastern regions of the plateau. The northern portion,

15750-588: The younger mountain ranges on the planet, consisting mostly of uplifted sedimentary and metamorphic rock . Their formation is a result of a continental collision or orogeny along the convergent boundary between the Indo-Australian Plate and the Eurasian Plate . The collision began in the Upper Cretaceous period about 70 million years ago, when the north-moving Indo-Australian Plate , moving at about 15 cm (6 in) per year, collided with

15876-451: Was most commonly experienced as the light emission from the inner surface of the cathode-ray tube in television sets and computer CRT monitors. In the SEM, CL detectors either collect all light emitted by the specimen or can analyse the wavelengths emitted by the specimen and display an emission spectrum or an image of the distribution of cathodoluminescence emitted by the specimen in real color. Characteristic X-rays that are produced by

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