纳米世界介绍——原子力显微技术（AFM）和电子探针

1、Introduction to the NanoworldAtomic Force Microscope (AFM)Scanning Probe Microscope (SPM)Force and its directionForce is a vector quantity. In addition to its magnitude, we need to define a direction. Force = 5 NFriction = 3 NA box is pulled along the ground with a force of 5N, a friction of 3N is a

2、cting on the box. Draw a cartoon diagram of this system. Force and its directionIn a coordinate system, xForcePositive force hereForce directed towards Right hand sidePositive force hereForce directed towards Right hand sideNegative force here Force directed towards Left hand sideNegative force here

3、 Force directed towards Left hand side0From Potentials to Forces: Example I020406080100120140-30-20-100102030r U(r)-15-10-5051015-30-20-100102030rF(r)Potential EnergyU(r) = kr2ForceF(r) = -d(U(r)/drF(r) = -2krFrom Potentials to Forces: Example -800-600-400-200020040060080010001200-30-20-100102030r U

4、(r)-140-120-100-80-60-40-20020-30-20-100102030rF(r)Potential EnergyU(r) = 0.3r2+0.1r3+0.2ForceF(r) = -d(U(r)/drF(r) = -2x0.3r-3x0.1r2From Potentials to Forces: Example Potential EnergyU(r) = exp(-r/5) 5/r6ForceF(r) = -d(U(r)/drF(r) =1/5 exp(-r/5) 30/r7 -2-1.5-1-0.500.51051015rU(r) -0.5-0.4-0.3-0.2-0

5、.10.00.10.2051015rF(r)Interatomic force vs DistanceForce rRepul si ve ForceAttracti veForce0N on-contactC ontactI nterm i ttent-contactWhy repulsive force has a positive sign while attractive force has a negative sign? Forces between atoms: Interatomic forceABrFrom As view, define coordinate system,

6、 to the right is positive to the left is negative ABrWith positive force ABrWith negative force RepulsiveAttractiveForces between atoms: Interatomic forceABrFrom Bs view, define coordinate system, to the left is positive and to the right is negative ABrWith positive force ABrWith negative force Repu

7、lsiveAttractiveInteratomic force vs DistanceForceSeparation rRepulsive ForceAttractive Force AtomForceForce = 0,Most favorable configurationForce vs Potential EnergyForce F(r)Separation rForce = 0,Most favorable configurationMinimum Potential Energy Most favorable configurationPotential EnergyU(r)F(

8、r) = - dU(r)/drSTM SpectroscopynSTM enables electronic spectroscopy of a single surface atom to be performed. nThe simplest form of STM tunneling spectroscopy involves placing the tip above a specific site on the surface, continually changing the tip (or sample) bias between two limits and measuring

9、 the change in tunnel current as a function of bias voltage. The height of tip is fixedThe position of tip is fixedChanging the bias voltage and measure the tunnelingcurrentSTM SpectroscopyFilledEmptyTipEnergy Levels of the Sample Why we call this spectroscopy? Because we are exploring the energy le

10、vels in the sample just like what we do in the case of atom when we study atomic spectrum. Empty StatesAtomically Resolved Electronic SpectroscopyApplied negative Bias Voltage to tipTunneling CurrentSTM Spectroscopy: Whats happening?FilledEmptyApplied negative Bias Voltage to tipEnergy Levels of the

11、 Sample Tunneling CurrentLow Bias. Nothing tunnel through, measure low/no currentSTM Spectroscopy: Whats happening?FilledEmptyApplied negative Bias Voltage to tipEnergy Levels of the Sample Tunneling CurrentSlightly higher negative tip bias. Tunnel into empty state of the sample possible. Tunneling

12、current starts to pick upSTM Spectroscopy: Whats happening?FilledEmptyApplied negative Bias Voltage to tipEnergy Levels of the Sample Tunneling CurrentHigher negative tip bias. Tunnel into the same empty state of the sample. Still measure tunneling current but magnitude does not change much.STM Spec

13、troscopy: Whats happening?FilledEmptyApplied negative Bias Voltage to tipEnergy Levels of the Sample Tunneling CurrentHigher negative tip bias. Tunnel into the same empty state of the sample. Still measure tunneling current but magnitude does not change much.STM Spectroscopy: Whats happening?FilledE

14、mptyApplied negative Bias Voltage to tipEnergy Levels of the Sample Tunneling CurrentSTM Spectroscopy: Whats happening?FilledEmptyApplied negative Bias Voltage to tipEnergy Levels of the Sample Tunneling CurrentSTM Spectroscopy: Whats happening?FilledEmptyApplied negative Bias Voltage to tipEnergy L

15、evels of the Sample Tunneling CurrentSTM SpectroscopyFilledEmptyApplied negative Bias Voltage to tipTipEnergy Levels of the Sample What kind of spectrum would you expect if you vary the tip bias in the spectroscopic mode? Tunneling CurrentApplied negative Bias Voltage to tipSTM SpectroscopyFilledEmp

16、tyApplied Negative Bias Voltage to tipTipEnergy Levels of the Sample What kind of spectrum would you expect if you vary the tip bias in the spectroscopic mode? Tunneling CurrentApplied Negative Bias Voltage to tip STM ImageSometimes during the scan, substance become attached to the end of the scanning tip and result in bad images. SEM In SEM, you have secondary electrons (SE) and backscattered electrons (BE) emitted from the surface, how do you distinguish between the tw