13. What is considered a weakness of scanning tunneling microscopy (STM)?
A. The inability to move and arrange atoms to create a design
B. The requirement for a conducting surface to work properly
C. The inability to apply this technology to biology
D. All of the above
Correct Answer: B. The requirement for a conducting surface to work properly
Scanning tunneling microscopy (STM) requires conductive or semiconductive samples because it measures quantum tunneling current between a sharp metallic tip and sample surface; insulators prevent current flow, making this its primary practical limitation.
Option Analysis
A. The inability to move and arrange atoms to create a design
False weakness—STM excels at atomic manipulation; IBM famously spelled “IBM” with xenon atoms using STM tips in 1989-1990, proving precise positioning capability.
B. The requirement for a conducting surface to work properly
True weakness—STM demands conductive samples (metals, semiconductors) for tunneling current; non-conductors like glass, polymers, or most biological tissues require conductive coatings or alternative techniques like AFM.
C. The inability to apply this technology to biology
False weakness—STM images biomolecules (DNA, proteins) on conductive substrates or in UHV; hydrated biology needs AFM, but STM works for dry/dehydrated biological surfaces.
D. All of the above
False—only B qualifies as a true limitation; A and C represent strengths or niche applications.
Introduction to Scanning Tunneling Microscopy Weakness
The main scanning tunneling microscopy weakness is its requirement for conductive samples, limiting applications to metals/semiconductors while excluding insulators—crucial knowledge for GATE Life Sciences nanotechnology sections.
STM Operating Principle
Ultra-sharp tungsten/Pt-Ir tips (atomic radius) raster-scan ~0.5 nm above surfaces in UHV; tunneling current (10 pA-10 nA) maps topography via exponential distance dependence (I ∝ e^(-2κd)).
Limitations Comparison Table
| Limitation Claim | True/False | Reason |
|---|---|---|
| Can’t move atoms | False | IBM logo demonstration |
| Needs conducting surface | True | No tunneling in insulators |
| Can’t do biology | False | DNA/protein imaging possible |
| All above | False | Only B valid |
GATE Life Sciences Tips
Scanning tunneling microscopy weakness questions target conductivity requirement—distinguish from AFM (insulators OK). Memorize: STM = conductors only. PYQs pair with TEM/SEM limits.
Overcoming Limitations
Atomic force microscopy (AFM) extends nanoscopy to insulators/biology via mechanical cantilever deflection. Hybrid STMs use conductive coatings for mixed samples. UHV vibration isolation remains universal challenge.
