跟读练习: Intro to Scanning Electron Microscopy (SEM) - 通过YouTube学习英语口语
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Welcome to the Technological Institute at Northwestern University.
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Welcome to the Technological Institute at Northwestern University.
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Today we are going to tour the Northwestern Atomic and Nanoscale Characterization Experimental Center,
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or the Nuance Center.
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We're going to start in the EPIC lab,
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which stands for Electron Probe Instrumentation Center.
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Here at EPIC, scientists from all over the world use high-powered electron microscopes to characterize a large range of different materials.
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Scientists such as geologists who study rocks,
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biologists who study cells, and even scientists who look at individual atoms and teeth use these electron microscopes.
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But what is an electron microscope?
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How is it different from a traditional light microscope you see in labs all over the country?
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What even is a microscope?
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Let's take a tour of the EPIC facility to learn more about the vast and growing field of microscopy.
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So what is a microscope?
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Let's dissect the word.
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The root scope means to watch or to see,
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and the prefix micro means extremely small.
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Together we can see that a microscope is an instrument
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that allows someone to see a very small object and comes in many shapes and sizes.
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The microscope you're probably most familiar with is an optical or light microscope.
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It was developed in the 1600s by scientists who made glasses.
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are made out of glass lenses that bend light.
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This makes objects seen through a lens look much bigger or smaller than they actually are.
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Telescopes, binoculars, microscopes, and cameras are all examples of objects that use glass lenses.
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In this microscope, a light source travels through a series of glass lenses.
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The first lens is the eyepiece lens.
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This makes the image bigger that's produced by what's called the objective lens.
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The objective lens is the highest power lens that's closest to the sample.
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Some microscopes have a condenser lens which concentrates the light beam before it travels through your sample.
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The sample is placed on the stage above the condenser lens and below the objective lens.
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Light then travels up through the lenses reaching your eye.
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What we're looking at now is a fly's wing in the light microscope.
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You can start to see the texture
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and even the hairs on the fly's wing that you couldn't see before with the naked eye.
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With the objective lenses on an optical microscope you can magnify the object up to 2,000 times.
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The way this microscope works is very similar to the microscope you have in your pocket.
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Light is energy that travels in waves of different wavelengths ranging from about 400 to 700 nanometers.
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This is visible by the human eye.
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Using light, we can make objects appear thousands of times larger than they actually are.
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But what if we want to see something millions of times larger?
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Something like a cell or a virus?
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Another form of energy comes as an electron beam.
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An electron is a very small negative charged particle that is part of an atom.
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A beam of electrons has a wavelength several orders of magnitude smaller than visible light.
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In order to use an electron beam to create an image,
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we have to use a special microscope called an SEM,
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or a scanning electron microscope.
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Here is one of our SEMs.
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It has an electron source at the top of the column
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that travels through the microscope column itself and into the specimen chamber where the image is created by a series of detectors.
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You cannot bend an electron beam with glass so you have to use special electromagnetic lenses.
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The lens set up in electron microscope is very similar to a light microscope.
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It has a light source or an energy source at the top,
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a condenser lens, an objective lens,
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and the sample is kept in a slightly different spot inside the specimen chamber.
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The image you see from light microscope is projected directly onto your eye.
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The electron microscope image is processed and displayed on a computer screen.
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An electron microscope sample is prepared on an aluminum pin stub using special carbon glue or carbon tape.
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We then coat the sample with metal,
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something that the electrons can pass through like a filament in your light bulb.
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If the electrons cannot pass through your sample,
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charging artifacts can make your image look really messy.
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Adding a layer of metal can help dissipate the charge buildup on the surface of the sample,
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making your image look much better.
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Once the samples are ready,
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they are inserted into the stage inside the SEM.
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Inside the SEM you can see the special detectors we use to create the image,
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as well as the objective lens pull piece that houses the objective lens and where the beam comes out.
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Now we can close the door and pump down the chamber.
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We have to remove all the air molecules inside the specimen chamber.
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If the electron beam interacts with air,
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it will scatter and not make it to the sample.
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By removing the air molecules,
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we create an unobstructed path directly to the sample surface.
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Now we are ready to go.
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Let's turn on the beam and see how this works.
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We move the stage under the pole piece to move around the sample.
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But how is the image being created?
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As the beam hits the sample,
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it interacts and creates secondary electrons that get sucked up by the secondary electron detector inside the chamber.
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As it scans across the sample,
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the signal is collected for each point or pixel and creates your image.
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This is the same fly we were looking at before.
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Unlike light microscopy,
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the signal you see in this image is from the surface of the specimen it does not travel through the specimen.
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Now we can see details on the wing that we couldn't see with the light microscope.
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Look at the hairs on the surface of the fly's legs.
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With an electron microscope like this SEM,
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you can magnify the image up to one million times.
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This is much greater than the 2,000 times of an optical microscope.
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This is the fly's leg.
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We also saw this with the light microscope,
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but what we didn't see are these hairs on the surface.
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The electron microscope allows you to achieve resolutions that you are unable to see with a light microscope.
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Sometimes the charge buildup on the surface of a sensitive sample can make the sample move.
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Here's the fly's compound eye.
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Have you ever seen this before by looking at a fly with the naked eye?
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Can you guess what this is?
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These tiny squares are about 1 micron wide.
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That's 1,000th of a millimeter.
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And makes up the pixels in your cell phone camera.
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Finally, this is the back end of a spider.
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It almost looks like the texture in your fingerprint.
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Another type of signal ejected from the specimen during the electron beam interaction are x-rays.
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With x-rays, we can determine the chemical composition of a material, like this penny.
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You can see that this penny is composed of copper and zinc.
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Since 1982, pennies were no longer made of pure copper and were made of copper-coated zinc.
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Have you ever noticed that Abraham Lincoln is sitting inside the Lincoln Memorial on a penny?
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Some other electron microscopes have an ion beam that can mill away your sample or deposit metal on the surface.
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So what is the difference between a light microscope and an electron microscope?
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The major difference is that light microscopy uses light as an energy source,
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whereas electron microscopy uses a beam of electrons.
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Light microscopy is easy to use,
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easy to prepare samples, and you can view live specimens.
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You can also see them in color,
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but you get much lower resolution and magnification.
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With an electron microscope you get high resolution,
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up to 1 million times magnification,
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with a larger depth of field,
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but you only get black and white images.
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You also have to do more involved sample preparation,
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and you cannot view live specimens.
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You also need an electrically conductive sample.
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The invention of the electron microscope changed the world of science forever.
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It is used in many fields such as biology,
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medicine, chemistry, geology, material science, and even forensics.
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Thank you for visiting the EPIC lab at the Nuas Center.
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We hope that you'll come back sometime and explore inner space with us.
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背景与上下文
在这段视频中，主持人带领观众参观西北大学的技术学院，特别介绍了原子和纳米尺度表征实验中心。视频深入探讨了电子显微镜的工作原理，以及它与传统光学显微镜的区别。主持人通过生动的例子，帮助观众理解显微镜的基本概念及其在科学研究中的重要性。这段对话富有信息量，适合想要提升英语听说能力的学习者。
日常交流的五个关键短语
- What is a microscope?（显微镜是什么？）
- It allows someone to see a very small object.（它让人们能够看到非常小的物体。）
- How is it different from a traditional light microscope?（它与传统光学显微镜有什么不同？）
- Light travels through a series of glass lenses.（光通过一系列玻璃镜头传播。）
- We need a special microscope called an SEM.（我们需要一个叫做扫描电子显微镜的特殊显微镜。）
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